nexus 7 volume keeps muting
As said in the comments it looks like your volume button could be broken. A good way to check if this is the case is to disable it. There are some great threads on how to do this so I will list a few here that you can check out. If the case is that the button is broken, let me know in a comment (on this answer) and I will help you out.

Threads:

http://forums.androidcentral.com/lg-optimus-g-pro/284907-how-disable-side-volume-buttons.html

http://androidforums.com/droid-support-troubleshooting/33602-disabling-volume-button.html

http://forum.xda-developers.com/showthread.php?t=2125810