I've added "sounds" that others have made to the game. I've never created anything from scratch or transposed anything from another format myself. As long as you stick to the format and specs for the original wav files though, and insert them into the correct zip with zero compression they should work.
You could just replace stock files (keeping the new ones the same name) with whatever you'd like.
Here's the properties for ABS.wav as an example. Match any new music files that you make to these and you should be good to go.