Oh, this is one of those things I went back and forth with and asked a LOT of questions from other creators and readers about!
1) Do you add a song for each chapter? Why or Why not? Not all of them no. Nope. No. I add music to my bonus chapters and I added one to a character bio I posted. While I always write to music, having the right score is... really hard (I'll go into more later). My bonus chapters are meant to add a bit more depth to the story, even if it isn't in the actual plot line, and they tease small, important moments.
I don't put them in every chapter. It's just not feasible to me. If you consider how long it takes a reader to go through an episode, you want the music to coincide with the reading experience and not clash against it, and most songs are actually longer than it takes a person to read the episode.
2) If you do, how do you find/choose the song? I always go instrumental so that the lyrics don't detract from the reading experience. Even then, I usually have a song in mind (and one that I probably wrote the piece to). Still, I go back and start the song at the beginning and read through the episode at different speeds to make sure the song rises/falls with what is happening in the piece itself, and I try to make sure the song ends roughly the same time as a read-through would.
I think with a comic that didn't have dialogue, you could easily go with a song that has lyrics but for novels... probably not the best route to take. In my opinion. Tapas doesn't autoplay music and you can only listen to it if you're reading on desktop, though, so if your reader doesn't really want to listen to the song they don't have to. It just would seem like a waste of time to me to put something that couldn't actually be enjoyed while reading the piece, if the reader chooses to.
3) Do you like novels that have songs for each chapter? Do you listen to them as you read the chapters? I... hm. This is hard to answer. If a novel has a song for each chapter, I'm less likely to listen to each one than if they were used more sparingly. Even when used sparingly, I'll sometimes play them only to find the music clashes with what I'm trying to read. So. I think it is a great value add but it can be done poorly. But since Tapas doesn't autoplay music, again, it doesn't take anything away from the experience of the story either way. People who want to listen will, people who don't won't.