LW's media player is not the most reliable. LW's 1st priorities are for searching & downlding & that's where most of its energies are spent. So if it's too busy then sometimes the media player suffers. What I'd suggest you do is disable the LW media player so that everytime you preview or launch a song, your default audio player launches instead. Go to LW's menu Tools>Options>Player & untick option box & press the Apply button.

If you'd like a reliable player that will play all types of mp3's & has in-built codecs & is free then check out QCD: http://www.quinnware.com/

If your computer can handle it, another player that is good, & arranges all your music into artist & albums sub-folders is iTunes: http://www.apple.com/itunes/download/

The above 2 players do not contain spyware like WMP does. WMP can not always be trusted. But if you're in love with it then stick with it.