One of my customers raised an interesting question.  They wanted to play WAV files from their Voicemail system... and it wouldn't.

The reason is - we don't support WAV files on Windows Mobile Media Player 10.  (See 5.3 at)

http://www.microsoft.com/windows/windowsmedia/player/windowsmobile/faq.aspx

By default, you can play files in either Windows Media or MP3 format (this includes files with the extensions .asf, .wma, .wmv, and .mp3).