I've had the occasional instance of this bug recently. Xion is playing a bunch of mp3s quite happily when it just stops and can't resume playback. The player is still in the play state and can be paused, but theres no actual progress or output. The only way it can be started playing again is to close Xion and relaunch it.
It doesn't crash and there are no error messages on the desktop or in the events. Most times this has happened has been when playing a repeating list of mp3s so maybe thats the cause.
I can't remember this occuring when Xion is not visible, so it could be a skinning issue.