I haven’t used an iLive or tried this on the GLD, but the iLive can probably do it. Go into the compressor settings for the music channel(s) and look for something like “sidechain” “key input” “detector source” or something like that. Set this to be his mic.
Edit: I’ve just tried this in the iLive editor (1.83 mind you) and it’s obvious how to do it. Towards the top left it says “Sidechain” with a drop down menu below it. From this drop down menu you can choose the channel you want to provide the key input for the compressor. Note that the channel providing the key input must be in the same block of 8 channels as the channel being compressed.
Edit again: …and there’s an “IN” button below the sidechain section you’d need to press to activate this.
Chris