nexus 7 volume keeps muting

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http://android.stackexchange.com – At completely random times the on screen volume slider appears and mutes the sound. Also the slider just stays on screen over anything I'm doing (as it is now) and will not disappear. Sometimes touching the volume rocker switch will work, often it doesn't. So even if I don't need any sound the slider is on screen interfering with whatever I'm trying to look at. Rebooting doesn't help. This occurs whether any apps are running or none and there is no way of telling when this will happen. Also if I do want to listen to something the slider will just appear and return to mute, so if I try to turn (HowTos)