I have 2 tanks in my living room - the exact same room where I have my home theatre system. I have actually watched the fish when a movie is on - and they don't behave ANY differently to when there's no sound. I think a fish get accustomed to the living environment they are in - just as they they would acclimitize to the water temp, chemicals, etc - they acclimitize to the noise levels. You guys should hear how noisy it could be with a lot of SCUBA diving boats roaring overhead when we dive, and with the bubbles also making a HUGE noise, no fish really stress out or scatter.
I can imagine a new fish that's JUST been added to a tank, being skittish and a bit stressed out by the noise - but after a few days they see that there's nothing that's going to eat them, they then relax and don't mind the noise anymore......
The only things that make my fish scatter is sudden movements right in front of the tank - as well as kids banging against the tank glass - not the noise, but the sudden movement makes them stress up a little bit...