My bad, looks like I posted this in the wrong section!
I don't think anyone is picky about where you post, as long as you don't post in *all* the sections under the assumption everyone looks at only one section...
Seems like the problem has to be one of two things:
1. Cable provider is compressing the audio more than before resulting in heap o' distortion (seems unlikely at first since the trend is towards less compression rather than more, but stranger things have happened). The fact that it only happens with on demand videos is *really* suspicious.
Guessing you have already done internet search with <name of provider> and "distort"... if not that would be a good first step.
2. Signal level from the cable box is higher with on-demand video than with other sources, overdriving amplifier inputs and creating distortion. Do you have to turn down volume control when playing on-demand ? Guessing "no" in which case this probably isn't the problem.
It seems unlikely that a wrong setting could result in intelligible audio but with distortion, but I'm really out of touch with cable boxes (no cable out where I live) but I guess it's a possibility... so:
3. What 2x6spds said