Switch to Timeline mode.
Right click the clip and select 'Split Audio'
Press the delete button to delete the audio part.
Add the music of your choice to the production library
Click and drag the audio clip to the Music Track
COMPAQ something or other with 500Gb Boot Disk, 1Tb external Firewire Disk, 4Gb Memory and a super duper sound card, 19" TFT Monitor etc etc etc
Sony TRV33e : Sony TRV310e : Canon XM2
Sony Vegas Studio editions to v11
Remember, there is always more to learn than there is to teach.