It could be due to a few reasons. My 1st guess is that LW is too busy for the media player to work properly. That happens sometimes. So some people disable their LW media player so their default media player works instead. Go to Tools>Options>Player & untick option & press apply button. But still you'll need to wait til next time you open LW for this to take effect.

If you'd like a player with in-built codecs & plays all then get QCD: http://www.quinnware.com/ & to make this your default player use the same principle here: Setting Default video app OR you might like iTunes instead: http://www.apple.com/itunes/download/

* There are some corrupt files out there so just be aware: T3 Spammers; mp3 Bit-Rates Versus File Sizes

* To find your music go to your LW Library window & select the saved files or Shared folder.