Odd "Secondary Audio" Problem in Total Media Theater

[JayX]

I put my copy of 'The Hangover' in the other day, to see it's Picture in Picture commentary mode, and even though the software confirms it's playing an English secondary audio track and I can see the PiP video feed... I can't hear it. The normal movie audio plays as it should.

I have the HTPC hooked up to my receiver using an optical connection, and the software is set to send everything over S/PDIF and this has worked fine for everything else.

Anyone had this problem, or know of a way to sort it? Can post screenshots of the Dolby/TrueHD settings pages when I get home if useful.