If I am listening to music when I write the genre I choose is classical. Writing each book could be viewed as a classical composition as well, combining every player into a well-rounded, beautiful blend of melody.
First are the soloists - your main characters. Each must speak with their own voice and yet blend harmoniously into the piece as a whole. I think about each character's vocabulary and speech rhythms. Is this character well-educated? Are they plain-spoken or crafty? Nervous or confidant? Answers give me the right notes.
Surrounding each of your soloists are the other members of the orchestra - your background characters. Each has an essential role to play but can't draw too much attention to themselves. Tricky. I aim for their voices to be heard for a moment but not to be so unique that the voice will be missed.
No symphony could be played without a conductor - the one who puts their mark on every player and makes every creative decision - the author, if I continue this metaphor. The author's voice is heard throughout the piece and must be distinctive, yet let the other players be heard clearly.
Nobody said this was easy; but it certainly can be a satisfying experience to read it all come together and hear each book's symphony.