-First of all, if the music video is in .wma format, you should do the following.
-Next, you need to take it from your directionary to Sony Vegas' Video Track (The audio track will appear automatically).
-Then delete the video track.
-Render the audio as an MP3, WAV or whatnot.
There you go!
Or, if the video is in .avi format, you'll do the same, except you don't have to delete the video track, since no video track will appear
-Rothicol