It involves putting a compressor plugin on your music that has a “sidechain” or audio key feature. While the compressor is placed on the music, the sidechain is set to listen to the dialogue track. Duckers also do a similar job in a similar way but are arguably designed better for the task.
So that’s where to put the compressor, but setting it can be a little more tricky.
Make sure the compressor is listening to the sidechain. You will want to audio to drop pretty much immediately, so the attack should be set pretty fast, maybe around 3-5ms. If sounds to abrupt when dropping the audio, lengthen the time, if it’s not quick enough, lower the attack.
The release should be set to a long time, long enough so the music gently fades back in, but not to quick. Try around 1sec and lengthen or shorten it to taste.
Next set the ratio to about 4:1 which may just be displayed as plain 4. This is a place to start but will most surely need tweaking as well. I’ll get back to that.
Now as the music and dialogue plays lower the threshold until you hear the music dropping in volume. Keep lowering the threshold until you get the volume drop you want. Adjust the attack and release to taste.
If the music sounds too suffocated while being reduced, reduce the ratio. If it sounds relatively soft enough, but still seems to be a bit too distracting, increase the ratio. Readjust the threshold to taste, the lower it is the more volume reduction.
Not that the threshold is usually measured in dBFS, which is the same scale as the audio meters display. This just means don’t be fooled by the big numbers, they are negative values. So lowering a threshold will make it go further into the negatives, while increasing it will bring it closer to 0dBFS.
I probably wouldn’t increase the ratio past 10:1, or 10, and a ratio of 1 means the compressor really isn’t doing anything. For this purpose, 3 is probably the lowest you would want to go. Your magic zone should probably be somewhere in there.
This might all seem a bit confusing, but do it one step at a time. There should be plenty of videos on YouTube as well to help you to sidechain a compressor to duck music.