You can use .wav and MP3s for the Pre-Recorded files.
If you upload a .wav file, it has to be smaller than 4M and a 8Khz, 16 bits, mono.
If you upload a .mp3 file, it will be converted to a .wav file for you. However, the -resulting- .wav file must also be smaller than 4M.
If you have any trouble please let us know.