Originally Posted by ijansch
If I play this through AirPlay, I hear a second frequency going up and down, between 5khz and 6khz.
It's hard to be certain -- especially since I'm basically unable to hear anything after 4kHz. But with my headphones, I get what you describe -- suddenly, something, that gets loud then quiet, between 7 and 8 kHz for me. Since I can't hear anything >4kHz, I guess that sound is "another frequency" as you note.
I bet this is simply an artifact (e.g. aliasing, probably, or nonlinear effects due to quantization) somewhere in the chain that manifest only when you use AirPlay, and me only with my headphones.
That being said, I don't think this explains why Sam is clipping at low volumes.
Try to bisect the problem... E.g. listen to the same source but without Airplay (e.g. directly on an iDevice or AppleTV wired to your AVR/amp). Is it better/different? If so, then focus on the source. If not, then Airplay is not the culprit. Proceed systematically to isolate the source of the percieved problem. You may find it's something trivial (e.g. output volume of a device set too low or too high, etc.), or a bad connection.
E.g. on my computer, setting volume to 100% clips some content. I'm better at 90%. Also, for some reason, on my bi-wirable towers, the speaker terminals were loose and the connector plate that bridges the bottom and top terminal was not making contact properly, so I was lacking the highs on that speaker. Or the RCA subwoofer cable would detach due to me moving the AVR. Etc.