regarding to the muting situation
hey there,
i stumbled upon this site while googling the related issue concerning the volume commands on my imac.
i too was having a problem not being able to un-mute the computer. the f10, f11, and f12 buttons were all unresponsive with the volume display repeatedly showing the "muted" indication. i personally use a different output source other than the built in speakers. for some reason, this causes the conflict. a simple fix is to go to your computer system preferences and switch your sound output back to the built in speakers. only then can you unmute and adjust the volume from your computer itself. then simply switch your system output back to your interface or speakers etc.
..or at least this was my solution.
i've only been using macs for about two weeks now and was never that computer savvy to begin with. this is probably a no brainer for some people.