Something else is using your sound card, and it appears that your soud card can only handle 1 or 2 simultaneous wave playbacks (mp3s use wav output). Close any other applications that might be using it. I'm almost sure that LimeWire doesn't use the soundcard at all, so something else is open that needs to be closed. Check your system tray for open programs. You can right-click them and close them should you find any.

Hope this helps!