You need a program that can compress waves.
I'd reccomend you to use 'dBpowerAMP Music Converter' (you can find it in google).
It's free, and can convert wav to mp3, mp3 to wav, choose the encoding level in mp3 and in wav you can choose stereo or mono, how much Hz, and how much bit (so that you can udjust your sounds to either be high qualinty but big or low qualinty but vrey small).
Starcraft can run 16 bit and 8 bit.
Infact, you rather use 8 bit, since if you use 16 bit it automatically compreses it to 8 bit anyway.
If you insert 8 bit it tells you that it won't compress - because it already is compressed.