This is new in 1.3.3 so make sure you have at least that. Let's get started. First, open up audacity and import your audio. In this case I'm picking an mp3:
Now import your commentary. I'm just recording something from my mic. Make sure this track is placed below the game audio. In my case I'm not bothering doing any cutting, noise-removal or amplifying of either track - in reality you would do this first. So however you do it, put your commentary below the game audio:
Now select the section of the game audio you want to auto-duck. Don't select the commentary audio or audacity will bitch at you with a really bizarre error message about you missing a control track (that's your commentary), which stumped me for a bit:
Now go to Effects -> Auto Duck, and it'll pop open a window to configure the effect. Here you can control how much the game audio is reduced by, and how much. You can also configure how sensitive the effect is - ie. how soon after you stop talking will it try and bring the game audio back up. You can also choose how fast the game audio goes in and comes back.
Now hit OK and wait until it processes. When it's done your audio should look something like this:
And you're done! Feel free to play around with the effect. You might want to do this on a small section of the audio since the effect can take a while to process each time.
Comments welcome, I haven't used this much myself since I tend to just always speak over the game except for cutscenes, and I prefer a uniform volume rather than it coming in and out very rarely.
An aside, but here's a demonstration of what happens when you amplify a waveform up to clip, and then back down by the same amount (12dB as it happens each direction):