If you're planning on making sounds for Peep, I suggest looking into a program called Goldwave for sound processing: goldwave. If you're processing event sounds, which tend to be higher in pitch by nature, I suggest running a high-pass filter at 1 Khz, to get rid of a lot of the low end noise. Finally, please remember to normalize your sounds so that they have maximum volume.
The sound format used by Peep is PCM raw signed-16 bit stereo at 44100 Khz (CD-quality), which has the extension .snd. To associate sounds with a single event or state, simply put all the sounds under a single directory in the sound repository.