Henjinx
It's still an error. :/ If it says mute all sounds, it mute all sounds. Not to sound like a snot nose kid, but bugs like this show up all the time, and after a couple of months I am just tired of it.
Thats actually been a problem since... well, when zOMG came out unfortunately~
There is a somewhat work-around (if you run Windows Vista at least); when the zOMG window is open right click the volume icon and click "open volume mixer"~
Then, turn down the volume for "Plugin container for Firefox" (in my case specifically)... and it should work~
That said, if you are trying to... watch a youtube video while playing, you'd have to use different browsers... but yeah~