Why woujld you want to do that? The RIO will play MP3 files. To make them smaller just use one of many converters (MusicMatch for example) and save as a smaller bit rate. I have found that 48 kbps works fine. Even 16 is tolerable (Note that Audible format 2 is 8 kbps and format 1 (the RIO 500 uses format 1) is 5 kbps.