Are there any surround sound settings on your receiver that are turned on? If so, try turning them off.
Also, are you using the optical out? If so, try using the RCA jacks instead. It could be that the changes in the feeds you talk about might be causing a switch from 5.1 surround to 2 channel stereo, or some other type of glitch in the audio stream on the optical out, confusing the receiver. (Something that might be interesting to try, get the TV in it's echo mode, then turn the receiver off and back on (or change sources on the receiver) and see if it fixes the issue.)
Also, make sure you don't have any "entertainment" mode set (menu, setup, entertainment), although I don't think it would have any affect on the sound when you the the internal speakers off).
This sound crazy, but it is absolutely true.
(My system consists of the TV (LN46A550), and a Panasonic DVD recorder, and Panasonic Home Theater in a Box. The Samsung TV is the only change to the system)
I have the on board speaker turned off on the TV so that I can use the much better sound from my Panasonic HTIB. That is how it has always been, with two previous TVs.
I have no cable. Just rabbit ears. I get all local channels without much problem, other than tweaking the antenna occasionally.
BUT I have a very strange audio problem with my TV (LN46A550).
On two channels, both major network affiliates, and only on those channels, the sound switches to an echo chamber effect under very specific circumstances.
When the broadcast goes to commercial, or switches between local and network broadcast the sound is like an echo chamber. If I momentarily click to another channel, and then back, the sound it restored to normal.
If I watch programs using the DVD recorder's tuner, this does not happen.
I called Tech Support, and was told that unless the problem was on all Channels, it was not the fault of the TV. I asked whether or not there might be a software update for my TV, and the tech said that if I updated the software I would void my TV's Warrantee!!! He was NOT helpful at all.
Anyone have a clue what might be causing this? All feedback gratefully recieved