Videos that have multiple audio tracks can be challenging to manage using the Volume Envelope tool alone. To cause a background audio track to soften when a particular video clip plays, use the Ducking tool. Only video clips on the Primary Timeline can be manipulated for ducked audio tracks. These are video clips imported using the Media button at the top of the Media Wheel.
To mark an audio clip as background audio, tap the audio clip to select it and enable Ducking.
Next, tap the video clip in the Primary Timeline you want to adjust the background audio clip against. Then tap the Mixer tool. A new slider, to the right of the master volume slider will be available. Use this slider to set the background volume for this particular video clip.
Lowering the Ducking slider will soften the background music while the video clips plays. Raising it does the opposite.