Maybe the best idea would be for your girlfriend to get Ventrillo, then it may be possible to change different audio conversations using a keyboard shortcut?
As an alternative, there's a mute keyboard shortcut in
Skype, so you could have a voice conversation with your girlfriend on that while gaming? Actually, even better Skype add-on
Skype for Power Gamers (S4PG) allows push-to-talk.
There's a built-in Voice Clip feature in Windows Live Messenger which, when you hold down F2, sends a voice clip. However you have to have the conversation open as far as I know, so it wouldn't be a solution to your problem. I don't know of any way to mute an audio conversation in WLM with a keyboard shortcut unfortunately.
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