Are these actual high def shows with this issue? The only time I've seen something like that is when there was a problem back at the source - ie. the originating network (and I've seen all sorts of things, like swapping rears with fronts, multiplexing dialogue onto *all* channels and so forth). Dolby Digital signals are usually quite hard to mess up after the fact.

Now if these were analog channels or SD shows, then I've seen that a lot. A number of years ago the local cable company "lost" right channel audio on one of the most-watched channels for about 8 months. I knew it wasn't a source problem, as antenna picked it up fine. They refused to awknowledge the issue, but it was eventually fixed.