you put a compressor on the background music with the sidechain input set to the channel your microphone is on, so when you speak the music is automatically pushed to the background and you can actually hear the mic.

What the other poster is describing is more commonly called "ducking". https://en.wikipedia.org/wiki/Ducking

The secondary audio (background music) automatically lowers when the primary audio (narration) is active. It allows the narrator to talk over the background music without being muffled by the music.

An automated way to do this is in audio production is by using the sidechain input of a dynamic range compressor. https://en.wikipedia.org/wiki/Dynamic_range_compression#Side...

Rather than having voice over and background music clashing at the same volume levels, the music level would decrease whenever the voice over signal is active.

Thanks! Confusing because I think "bitrate" when I hear "compression" but it actually refers to the volume level.

In this case, it's basically where when the presenter is speaking, the music quiets at the exact moment, so you can hear more clearly but still have the music.