If you're using LW 4 go tools > options > player. Tick the box to use LW's player > Apply > ok.

If you're using LW 5 go tools > options > My Library. Tick the box to use LW's player > ok.

LW's player isn't good - I'd recommend using a different player. You might need codecs for your WMP.
Or you could try VLC - it's free & has codecs installed to play most file types. If VLC won't play a file then chances are that the file's bad. Check the link below for info about codec packs & links to other players If you do install codecs, make sure that you only have 1 codec pack installed at a time - multiple codec packs may conflict with each other.
http://www.gnutellaforums.com/tips-t...p3-fixers.html