I can't select an alternate audio track.

Recorded shows, such as those in the My Shows list, always have only one audio track, the one they were recorded with.

You cannot change the audio track if you are watching a live show, but are not caught up to live TV. Catch up to live TV by pressing the ADVANCE button. Changing the audio program will clear the saved part of the show you are watching, so you will only be able to rewind live TV to the point where you changed the audio program.