Man... I have considered this a lot in my fantasy comic Whispers of the Past. Technically, the language the characters are speaking is a fictional language called Sunang, even though the dialogue is written in English for the readers. So that means a lot of lilting rhythms, soft Rs, trilled Rs, and generally soft pronunciations except for rare words/phrases.
So anyway, I imagine the characters' voices very specifically.
Izrekiel would have a very precise pronunciation, distinguishing every syllable very distinctly. This an uncommon trait in Sunang, but he basically speaks a completely different dialect than most of the other characters. In regards to his vocal quality, I imagine a strong tenor, which means, not very low pitched, but not a soft voice either.
Agatha has a husky voice. Her words are slurred slightly, which is common in that region of the country: blurred consonants, softer Rs, very round pronunciations. For a woman, she has a somewhat low voice, but smooth and lovely in quality. She sings sometimes, and her tone is very pretty.
Kelan, Agatha's father, has a similar accent to her, but his voice is very low, a deep bass so low that it's sometimes hard to hear. In regards to vocal quality, his voice is slightly raspy, not airy or breathy, but a little gruff, despite the friendliness of his demeanor.
Lord Ryukou (who is introduced several chapters later) has an accent almost identical to Izrekiel, very proper, a different dialect than the others in that region. His vocal quality is very gentle, a smooth, nearly monotone voice with little fluctuation in pitch. He has a soft baritone voice, a combination of commanding, friendly, and slightly breathy.
Lady Maya has a harsh mezzo-soprano voice, which means it is somewhere between high-pitched and low-pitched.. She has a distinct accent, but not quite as distinct as Izrekiel's or Ryukou's. Her voice fluctuates in pitch a lot, and she tends to exaggerate her words frequently.
Orato, who shows up waaaay later, has an ethereal voice, soft, indistinct, an ambiguous accent, and somewhat androgynous. Their gender is hard to place, if they even have a gender.
Saa has one of my favorite voices, because it is layered with many voices and vocal qualities, and I actually already have an English voice actor for him! He is also the only character who I imagine speaking in English. You can check out a voice sample here.
Anyway, if you are interested in learning more about the language of Sunang, you can check it out on my website.