You may need to run these meetings in the browser. I'm pretty sure I have run two meetings at one time, both in the standalone/out-of-browser flash player that automatically launches when you have the presenter add-in installed, but I'd have to test to know for sure. If you have to you could always just keep them in the browser by adding ?launcher=false to the end of your meeting URL.
]]>Meeting 1: Firefox
Meeting 2: IE
Meeting 3: Chrome
In the Windows 7 sound mixer thingy (in the system tray), you have the ability to change volume/mute/unmute per program.
It's a pretty clunky way of doing things if you have to do it manually like this. There could be a program that routes/mutes/unmutes the individual program audio output. If so, that's the way to go.
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