This article discusses how a lot of writers hear the voices of their characters:
I’ve been even more aware of it recently, as I gear up to become a narrator of my Cornish Detective series. I’ve ‘heard’ my main character and his detectives’ voices in my head many times, but actually expressing them with my vocal chords is going to be a challenge!
The novella I’m writing at the moment, set in Georgia after the Civil War, has made me contemplate how I’ll make my veteran soldier sound. He’s from Pennsylvania and has been warned by people he’s met to modify his accent while in the Deep South, to avoid antagonising the locals. On the other hand, he’s just met a cultured Mulatto ex-slave who makes him feel like a hick. This man doesn’t sound like a black slave, more like a professor of English.
Do you hear your characters speak?