Open the movie window, and click on the button "Audio X" (Audio 1/Audio 2...).
A new window appears where you can set the audio, but that's pretty much it.
If you want to label the track, that's probably not possible in DVD-Lab Pro and probably not even in the DVD specification :-) You can just set some attributes like "director's commentary", "alt. director's commentary", "for visually impaired" and "normal" but that's it