From various illustrations by Rowling herself, we have at least some idea of how she imagined most of her major characters to look. For instance, here's a piece of concept art she drew in relation to the first book:
But what do we know, if anything, about how she imagined them to sound?
What canon info is there on the voices of the major characters in the Harry Potter series?
In this interview with JK Rowling, she describes how she imagines the voices of some of her characters (Hagrid, McGonagall, Dumbledore, and Harry):
Lydon: Now I want to hear J.K. Rowling herself do ...
JKR: Do Dumble-, er - not Dumbledore - Hagrid!
Lydon: Either one ...
JKR: Well, Hagrid's sort of West-country - yokel - which is where I grew up - the part of Britain where I grew up, I didn't grow up in Scotland, I grew up on the border with Wales. So Hagrid's kind of ... 'Yes Harry Potter, sir' - like that - very slurred words - it's the accent English people always put on to sound stupid ... [laughs] Hagrid isn't stupid, but he's got that kind of very country - you know, way of speaking ...
Lydon: And how about Minerva McGonagall?
JKR: Very clipped, and very, very - quite upper-class and very brisk - like a governess [Lydon laughs] I - I - I can't do it, but I kind of see Dumbledore more as a John Gielgud type, you know, quite elderly and - and quite stately.
Lydon: And Harry himself?
JKR: I suppose he sounds like me. I always do my voice for Harry, when I'm reading to my daughter.
The audio of this section of the interview, including a minute or two of JKR reading aloud from her own book, is available here (Internet Archive; sadly the original link has gone dead).
