Answered here, somewhat:
http://www.amazon.com/gp/help/customer/display.html/?nodeId=...> Files must be music recordings in MP3 (.mp3) or AAC (.m4a, iTunes non-DRM files) format and must be less than 100 MB in size... audio recordings that are not of songs and non-audio files (even if in MP3 or AAC format) are not eligible for unlimited music space.
