I take it you are listening to a stereo source when this happens?
If so what you are hearing is the amps attempt to take the stereo signal and turn it into a surround sound. The same thing will happen if you use a prologic or neo setting.
Prologic or similar output mono audio contained within stereo audio via the centre speaker. A lot of the audio on the TV is mono in nature, this is especially the case with dialogue and most TV shows comprise of more than 80% dialogue with the rest of the audio made up of background music and sound effects. Pro Logic basically takes any audio that lacks a difference in phase and outputs it via the centre channel. If phase shifts are detected then this is used to determine where to send other sound, but the majority of TV content is dialogue based and has no phase shift.