Salutations!
Well I have read through your story and I though I'd give you some feedback as per your request!
The first thing I want to start with is, I actually found it quite charming that each of the characters had a last name that was musically related. (Harmony, Serenade, Lullaby, etc,) Now, granted, I know that if you were doing a more serious piece this could get in the way of the narrative but, with your story being more plucky, it actually works. It keeps your story upbeat and establishes a mellow atmosphere in that you're not taking the story drop dead serious, so the levity is well translated. I also like that you start off in the middle of action and a concert where atmosphere is the character more than anything, it helps establish your story and you lay early groundwork here that makes a nice preface for the tone of your story to come.
I will say that the writing was a bit hard to follow because it was written in a manner more befitting a movie script than a novella. I felt more like I was reading a screenplay or stage directions than getting fully immersed. This comes from the use of -s instead of -ed at the end of your words. This makes your narrative sound passive and as a result, your descriptions feel like rapid fire run-downs than actual descriptions. For example:
"As they scream the last number, the thunder of their voices is accompanied by the rhythmic booming of the band’s drums, the sound vibrating beneath their feet, followed by waves of electronic music grinding from a single guitar."
This section could've been a great way to establish the thick of the scene right from the off, however the -s endings make the narrative sound passive and as if it's in a hurry to finish it's thought so the next paragraph can come. This takes a reader out of the story because you almost feel like you're being shoved from point to point without the ability to absorb and revel in the scene because it's in a hurry somehow. This is just from a readers perspective, from my editing perspective, if I wanted to involve my audience I'd use -ed at the end of the sentences because it puts the reader in the moment. Example using the same paragraph:
"They screamed the last number, the thunder of their voices accompanied by the rhythmic boom of the drums. The sound vibrated beneath their feet, followed by waves of electronic music shredded from a single guitar."
Now, this is by no means perfect but, the difference in the action can make all the difference to a reader. The descriptions don't have to be purple prose, just targeted at eliciting feeling and emotion. You also have to be aware of the "Passive Voice" which is a chronic issue that crops up in everyone's writing, not just yours, haha. There are two ways that "Passive Voice" can ruin your narrative: by weakening your description and by adding additional words that aren't required for the description itself. Now, don't get me wrong, Passive Voice has a place and is very useful in that place, but when it comes to the kind of descriptions you're making here, it weakens the narrative.
For example: Your narrative has lot's of -ing words where -ed words would be better suited to keep the audience on board. Overuse of -ing becomes noticeable after a time and can wear out a reader. This doesn't seem to happen as much with -ed because, again, it's about being in the moment, showing versus telling. I will say this, you had a distinct lack of -ly words and that isn't a bad thing at all! They have their place too and can be great, but overuse of -ly words leads us to the same problem as overuse of -ing. However, you did not have both, which is awesome because that means you used more accurate words to make your descriptions and didn't need to rely on -ly. This is very refreshing to see from an editing stand point, so good on you!
Now, beyond the technical stuff, one thing that did take me out of the narrative was the beginning where Blaine Harmony is introduced three times. (You give him three introductory paragraphs/scenes):
-"In the audience, Blaine Harmony lifts his arms, fists pumping, welcoming his favorite performer along with the other ecstatic fans around him."
-"The feeling of a kindred excitement from the crowd as they sway, nod, and pump their arms with the music, fills one of the concert-goers, a young man named Blaine Harmony."
- " Blaine Harmony, now the drummer in his own band, “The Jupits,” puts his drumsticks down across the top of the floor."
After the first time you introduce him as Blaine Harmony, you don't have to call him by his first and last name every time. That feels clunky and derailed my focus when I was reading. I know that this was done because you have a theme with the last names, but don't you worry, we the audience will definitely remember it. Because of the levity you established, it makes you smile as you get introduced to the others and hear their last names, which makes the first time you met Blaine all the more memorable. That memorability gets toppled if you re-introduce him too many times. I just wanted to give you a heads up about it because, I've done bits where I write a paragraph later and make the same introduction twice, and if it weren't for another reader, I'd have never caught myself! Haha so I didn't want to leave you in the lurch about it.
Now, one thing I also enjoyed right from the off is Blaine actually seems like a fun person to follow from the beginning. Even if he's later described as being moody, you can see him really open up to the concert and crowd scene and how he sort of waxes romantic about this girl he's never met but can't forget. That's charming and I think it made a very good introduction to Blaine. I also love how naturally he and his band mates talked and interacted. I like how the dialogue flowed and these people actually felt like they knew each other and that's refreshing to find. Sometimes in the beginning of a novel even characters who are supposed to know each other, all feel like they're meeting each other for the first time. This didn't come off that way so I think you did very well there.
Oh! One thing I wanted to make you aware of: The transfer of thought from narrator is very murky a lot of the time. Example:
-“Oh.” She looks down, her heart beating faster – those beautiful light grey eyes, his blond hair, the freckles…! OMG, he’s gorgeous! Who is he? And that smile is, oh, wow! She looks up and sees him run his fingers through his hair, a gesture that makes him somehow even cuter."
It's very hard to determine whose viewpoint we're supposed to be following when scenes like this happen because the narrator is simultaneously inside and outside the characters head. This is also difficult because you have several characters who have view points and I can tell you I got confused and lost a few times because of transitions like this being unclear. The narrator sounds like a separate person from Vianne but then we're inside Vianne's thoughts as if she were the narrator. Here is a simple fix to help your readers determine what's happening when and in whose head:
-" “Oh.” She looked down, her heart beating faster – those beautiful light grey eyes, his blond hair, the freckles. He’s gorgeous! Who is he? And that smile is, oh, wow! She thought. Vianne looked up to see him run his fingers through his hair, a gesture that made him somehow cuter."
Now, you'll notice I didn't italicize this one. That's because thoughts are often italicized within a narrative so that a thought and the paragraph can be separated. This way the narrator doesn't blend into Vianne's thoughts and confuse the reader. I also got rid of extra words that slowed down your narrative. Often times words like "Even" or "Absolutely" are just extra and pump up your word count but don't contribute to the scene. Not every word is going to be like that but, if it's not a necessary word it runs the risk of gumming up story flow. To that end "OMG" feels clunky. I'm not saying that people don't think it in their heads but I did fall out of the narrative because in a way, "OMG" feels out of place somehow. This just might be a personal thing on my part so, I won't be nuts about it, but I wanted to give you the heads up about it derailing me as a reader.
I do have to tell you though: I love Kaia. Now, all issues with the narration aside, this girl has bundles of personality right from the second we meet her and she feels like a person immediately. This is fantastic writing on your part and it really shows off your chops in creating characters. In the brief few paragraphs we have with her to start off, she's breaking rules, being cheery, and getting back at her nemesis all with a tricksters natural flair and in half a Tapas page! I enjoyed her entire scene from beginning to end. I could read a whole book about Kaia just getting by being herself. She's a delightful character and she really shines. If there is one thing you really have in spades, its the way to make a character feel natural and interesting.
Honestly I like your novella so far. It's charming, your characters all have a sort of wit about them, and I find that even with some of the issues I've mentioned, I'd love to see where this goes. Please keep writing, all of the things I mentioned above are things each author does, and, if you're running into issues, editing always helps. As such, I've a tool for you to use (which I tend to hand out pretty liberally around here): EditMinion. This tool will help you to identify passive voice, weak word usage, and adverbs that don't flow. I've found it to be very helpful. Here is a resource about writing in the third person as well, as your narrative seems to go back and forth as to whether or not it wants to be in the third person so, I figured some examples of things could be helpful.
Phew, if you've managed to get to the end of this thing, you are incredibly patient! I do wish you luck with everything and I hope my feedback is helpful for you. It's already a fun read so, that's the best way to start! A little fine tuning here and there will just go on to bolster the good that already exists.
Cheers!