You can't! lol
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Instead use your default mp3 player. To do that go to LW's menu Tools>Options>Player & untick option box. Press the apply button. Now your default mp3 player will launch everytime you launch a song. If you don't have a decent player, here's one that plays every type of mp3 files & has in-built codecs:
QCD (a free player) You know how to change default apps don't you.