For speech files, are there any best practices for using WAV or MP3?
Well, WAVs are uncompressed, so using them will result in a larger game size.
320kbps MP3s will be smaller, and the loss of of quality will be so infinitesimal (as to be almost non-existent) that it makes no sense to use WAV. Heck, you could even drop to 128kbps, and you probably still wouldn't notice much difference.
Just make sure you keep a backup of the original WAV files.
Thanks.