A brief google search revealed some people saying it's a bug in WLM (2009 or 2011?) having to do with wma or mp3 files. They say you could try midi or wav instead. This post is over a year old:
"Works for me, as long as I'm not set to read messages in plain text
(Tools, Options, Read). What type of background sound is it? MID, WAV, etc.? What version of Windows are you using? Open your audio volume application to check the volume settings. If a Midi file, make sure that the Synthesizer volume is not muted or set too low. "
Keep in mind that 2011 omitted stationary so any sound that was part of stationary probably won't work.
