Just to update anyone who is interested, the problem was that my cable box's SAP setting was set to Spanish.

For the majority of channels, this made no difference. For a few of the HD movie channels, audio came out in Spanish and for a few channels, this resulted in no audio at all.

I decided to look into the spanish thing after watching 'Hitch' for 10 minutes in espanol. Once I found that SAP was set to spanish and then turned it off, the few channels which previously had no audio worked again.

Strange.