You can't REMOVE it in WMM per se, but you can mute the audio clip and add a different soundtrack or do a voiceover narration. To do, click the + button on the timeline that you used before to make the audio portion of your video clip visible. Now right click on the audio track and select "Mute." Presto. Now you can go back and drop the sound you want in your audio/music line.
Hope it helps!