Sonar converts to a wave file, as MP3's suck butt for sound quality
MP3's degrade sound quality with the use of data compression. Its in your best interest to mixand record in a wave file format. Preferably in the 32bit floating point and at least 44.1kHz or 48kHz. anything higher than 48kHz for Hip hop/ Rap is a waste of space and resources.. You do not want to be mixing in an MP3 format. The sound quality will suffer.
After you done, you can convert to an MP3 format of 320 bitrate or 256, but i would not go lower than 256 bitrate for any MP3's.
CJ