Is vsync on in games? My Radeon makes a weird high-pitch noise when it runs at too high of a framerate, like a physical thing on the card is making noise. This doesn't transfer to the audio system at all though, and goes away when the framerate is locked at 60hz or lower. Probably unrelated, but you should be running with forced adaptive vsync anyway on your Nvidia card.
I still say make sure the amp and PC are plugged in to the same circuit at the same point for testing purposes. The problem is far more likely to be sourced in the analog power world than in the digital software world. Brightness sounds like a weird thing the be related though. The PC doesn't care if it's spitting out 00 or FF for a brightness value (in 256 level hex values). FF's don't cost more in power than 00's.
The projector will be grounded to the PC though, and the projector may throttle the bulb up or down for different brightness values, changing the power draw and therefore the reliance on ground. In a modern house ground should all run back to the same box, but in an older house ground may be more localized. American power standards are terrible, and I have learned far too much about how frankenstein they are in upgrading my 1953/1980-combo power system to something that supports modern devices. Something you can legally do under US federal electric code and may want to consider is dumping ground all together. You can run two-pole power to a three-prong plug if the wall socket is a GFCI outlet (those switching ones you find in your bathroom). GFCI with two-pole power is actually safer for human life than three-pole standard plugs. I had to install GFCI outlets throughout the 1953 section of my house in order to legally plug three-prong devices in. That was a "first week" project for my livingroom last September when we moved in.
So, maybe you want to isolate everything touching the home theater through GFCI. Don't take this as legal advice of course. I know what federal code is. I don't know what your local city and Cali state code is. Like everything, federal is an absolute, then state, county and city layer on top of that. It's possible that your city requires anything newer than 1975 to have grounded plugs regardless of GFCI. For quick testing purposes though you can totally open your wall sockets and disconnect ground to figure out if moving to groundless GFCI will solve your problem without buying a half-dozen $12 outlets. Again, disclaimer: Make sure those outlets are re-grounded before you walk away from them, and don't play with household electricity if you aren't comfortable with the basic safety rules.
I'm comfortable with testing for live wires by grabbing them with my thumb and forefinger, knowing that the resulting muscle spasm caused by a live 120v wire will force me to let go. I know how to use a multimeter to test wires before I get to that stage. I also know not to mess around with 240v because that will cause some lasting damage if you are careless. I don't know your comfort level.
I thought you lived in Colorado. Did you move recently, or am I thinking of someone else?
EDIT: Now that I think further, I believe a guy I used to play COD4 with live in CO. When playing TDU you lived in CA.