Any time of MP3 should work, if not lower the bit rate, do 128 or lower, that should work. You'll need some audio editing software like http://www.4musics.com/wav-mp3-wma-ogg-converter.htm

That should help you lower the bit rate, and allow playback on your PSP. I don't think there's anything else to do. I don't own a psp, but I did put music on my friend's psp, so I know how to do it, and I don't recall converting music. VIDEOS on the other hand do require conversion, use psp video 9 for that.