We have a TV that's less than 5 years old that has lost its sound only on a handful of channels. Another TV hookled up to the same connection has no problems.

That's odd, I would try unplugging Power for maybe 30 seconds, hold power button maybe 10 seconds, release and plug back in as a first thing. Is their a Place in Menu to reset to Default settings?

Have you tried retuning the 'bad' channels?

Daniel