We recommend uploading files in 128kbps at a sample rate of 44.1khz (44100 Hz) STEREO.  Our system is not engineered to run off of anything else other than 44.1Khz Stereo files, and as a result, any other settings will fail.  Any files that are uploaded differently will not be supported.  We have only tested 128kbps files, but generally all 44.1Khz STEREO mp3s will work.