The Audacity envelope tool is allows the reduction of background music volume and can easily fade music down when your voiceover starts. While most voiceovers are done without background music, for your own voiceovers or podcasts this tool is essential. It's very easy to use, but very hard to explain. This video I found on youtube is the best in explaining use of the Audacity tool- Video is by Colin Gray
https://youtu.be/wP9bzByrE0I
https://www.youtube.com/watch?v=szIX5ZDenUo
The Voice Over Coach - Audacity Basics 1- Envelope Editing
The Voice Over Coach - Audacity Basics 2 - Envelope Editing 2
The Voice Over Coach - Audacity Basics 3 - Editing Envelope