creating fan-based dvd audio commentary track

I have an MP3 that is fan-based audio commentary. The commentary was not mixed with the actual audio from the original television episode. I would like to mix the fan-based commentary with the original audio. But it would be nice if the original audio came through at normal volume when the commentary audio is silent, and have the original audio at a reduced volume when the commentary audio is not silent. It would be preferred that this volume adjustments are automatic.

Do you have any recommendations? I have no audio engineering skills, so I need a rather simple procedure.

Thanks!
Glen Hein

Use Audacity 1.3.13 http://audacityteam.org/download/
Use the Auto Duck effect: http://manual.audacityteam.org/index.php?title=Auto_Duck

Thanks! Auto Duck looks like the perfect solution. Thanks again for the quick response! You are awesome!

-Glen Hein

I tried Auto Duck and it did exactly what I wanted on the first try with the default settings.

I would like to thank everyone who has spent their time contributing to Audacity.

-Glen

Thanks for the feedback. Glad it has worked out successfully.