What speakers? You would be able to continue to use the speakers in the projector if it has them but you would not want to. You would want all your sources going to the AVR directly and speakers connected to that.
Can be the AVR. Can be caused by a bad speaker as well. Wiring too. Any pets that might of chewed on a speaker wire?
I'd start by disconnecting all speakers and see if it happens, then connect each one at a time seeing if it happens after each speaker is connected until all speakers are reconnected.
Is it just for two channel music? Are there movies, sports, and other A/V content involved as well?
If movies, sports, ect..ect... are involved?
I'd probably just look at a budget modern avr and run it 4.0 with speakers set to large. I recently scored a Pioneer 1122K for under $200 on an amazon warehouse deal, so they are out there.
2 channel just doesn't do movie content any real justice these days.
Get the one that has the best throw for your setup. I'd bank no discernible difference can be noted from the different contrast ratio specs on them. The no VESA connection on the 760 would be a bit of a bummer for me though, really limits your choices on 3D Glasses.
4 Ohm stable budget AVR's can be difficult to find. Thus the no responses yet on this.
Hard to beat Parts Express subs at just over $100
Leaves pretty much for a 4 ohm stable AVR. Should be do-able even with HDMI switching.