In your audio piece, there’s probably a point where you want to lay music behind your voice. This will show you how to do that.
Step One: Select the section of music that you want to fade when you are speaking.
Step 2: Select the envelope button
Step 3: Place a keypoint where you want to the audio to start to fade and where it will return to normal volume.
Step 4: Set two keypoints further in that mark the lowest points the audio will get.
Step 5: Then drag the two inner points down to an equal level.
Step 6: Check the audio and adjust if needed.
This video helped me learn this technique