This is the most important part:

To automatically force an audio switch, do this: Connection -> Add -> Add command object -> VTS domain

You will get a white box. In it, you will put one of these two commands:

SetSTN (audio=0 )

SetSTN (audio=1 )

Create an audio menu, with choices for each soundtrack. Between each button and the movie (or next menu), place one of these two commands. For example, the first track is the movie audio. If they select "movie audio", then it will link to this command, which tells the DVD player to play the fist audio channel, and then continues on to the movie.

"Movie Audio" buttton - (draw link to) - SetSTN (audio=0) - (draw link to) - Movie/Menu
"Commentary" button - (draw link to) - SetSTN (audio=1) - (draw link to) - Movie/Menu

Keep in mind that the audio can stil be switched on the fly during movie playback as well.

While all this sounds tedious, it actually makes a little more sense once you start making the connections. Which is why I like the "visual" aspect of DVD Lab projects - it works the way I think.