This seems like a simple thing to do, but for some reason no one's been able to give me a clear answer at all. If I'm working with one stereo full mix track and one vocal track, how can I get the full mix to duck down when the voice over comes in? I haven't got any external hardware to work with -- just the base ProTools 8 plugins.
Set up a compressor on the stereo track (PT's own will do). Send the vocal track through a bus(mono) into the sidechain input of the compressor. Make sure to activate the sidechain, it's the knob with the key on it in the Sidechain section of the plugin.
Experiment with the knobs in order to get it to sound smooth.
Depends on what you are working on though. For bigger projects like commentary on a feature film it's a time saver bu not always the best choice.
I wouldn't recommend it for smaller things like ads or infomercials, then it's best to do a volume automation manually.
It can be hard to make it sound natural and smooth, bear in mind that in order to duck a signal it needs to reach the threshold first. So it'll never be as good as manual volume control.
Hope this helps.