To separate the audio from the video:
> In time-line view of WMM, click the + button beside the word Video
> Right-click the Audio track and Copy it. Then select Cut
Your entire video will be removed with the cut.. but don't worry, it's part of the process.
> Close the + Button beside the Video track
> Right-click on the Audio/Music track and select Paste
Now the original audio from the video is in a separate track and will play throughout the movie. But, you aren't done yet.
> Drag the original video into the Video track again from the Imported Media pane.
> Click the + button beside Video to open the options once more
> Click Mute from the drop-down menu
> Close the + button
This is how you separate the audio from the video so you can edit it separately.