The feature you're looking for is called Audio Ducking and can be achieved by applying a side-chain compressor to your background track or by key framing your background audio volume to lower when the other track is playing.
A side-chain compressor monitors the amplitude of
signal B and applies compression based on that to
signal A. So if
signal B will have audio in it,
signal A will be quieter for that moment.
Get a VST or AU side-chain compressor, apply it to your background track (
signal A) and set the other primary track as the side-chain input (
If you have multiple commentary tracks, either merge them to one, or apply the effect multiple times to
signal A but with different side-chain inputs.
The other method involves keyframing your audio track volume and is completely explained in this Adobe Forum post titled: Adjusting Audio Volume Levels