The main game I want to re-translate with this project is Drascula. The ScummVM team did a good job fixing some of the wrong English in the text, but now the voices don't match. Even so, the voices are so badly made because they were spoken by non-native English speakers. Many people turn the voices off to play the game, or just don't play it at all because of the voice over work.
So I had a look at the ScummVM forums and sourcecode to see if I could figure out how to change the voices. I found out that packet.001 holds all of the game's data, and it's compressed in ARJ format. The ALS files contain the voices, which are 8-bit unsigned RAW sound files. In audacity, I can record a voice and increase the speed to 460 and it seems to play fine in ScummVM.
I tested this with a clip from Foster's Home For Imaginary Friends where Cheese says "Can I have some chocolate milk?" I saved the file as D1.ALS, which is Drascula's first line in the game, and it played fine. :)
Now that I know how to change the sound files in-game, I can go forward with re-recording the English voices. :)