Hi Paul... so you've found Tools/options you should then see a list on the left:saving,sharing,speed,downloads,uploads,connec tions, PLAYER,shutdown click on PLAYER and in the top right hand corner you will see the message "use LimeWire mp3 player" the greyish box must have a tick / check mark in it, if not click on that box until you do see one. Then BEFORE you leave, left-click on "Apply" and then "OK" that should solve your problem.
I assume you are using version 2.2.1......
If that doesn't work try un-installing program, and fresh install, then when asked opt to use LimeWire's mp3 player.
Have fun.......