You didn't indicate which version of MEP that you are using.
In MEP2014, right click on the video object on the timeline, Audio functions, Remove audio object. You can put it back the same way.
This is pretty basic. I suggest that you look at the other audio options and try them out. Also, read the manual. I knew the answer, but searching for Audio in F1 Help would have given you the answer faster than posting a query here, and you would have learned a lot more.
Another, pretty much version independent, option is to first 'un-group' the audio and video (broken links icon) so you can simply select just the audio and delete it.
The two get imported on different tracts in older versions but with newer versions you may need to do a right click and pick Audio.Video on separate tracks from the Audio functions list.