Does the main volume control on the 768 change to the TV from the AV system when I run audio through the TV instead of the AMP?

If I understand your question corectly, the answer is no. However, what happens is this: if you told the harmony that you use your reciever to listen to the sound and NOT the TV, then just the volume keys are configured to control the reciever, and the rest of the buttons are for the TV control.

Hope this is clearer than mud.

OK Lets try this:
My DVD and SAT audio run through my AMP.
My Replay audio runs through my TV speakers.
When I change from watching Replay to SAT does the 768 volume control button switch from crontroling the TV to controling the AMP?

it should- when you set up your activity you tell it what your audio source is== receiver or tv