The QC 35 (mk II, at least) has nothing on the cord -- it's just a cord with different sized TRRS plugs on either end -- I don't think my QC 25 had one either.

As others, I've had a huge variation on aircraft type and crew and there is definitely volume control available to the crew (though it may only be accessible from the flight deck, not FAs -- on the MD10 [I know not a UA aircraft, but the only one I've officially played with its on the comm panel about waist high aft of the FO seat])

I've never foun it loud enough on my flights to be worried about hearing amage but I have either whipped them off my head or pulled the plug because it can be uncomfortable especially when some FAs like to drone on forever. On the 737s I think it's more the headphones do a much better job oflowering the noise floor than anything else and the volume is set to overcome all of the background (engines, wind, etc., etc.) for the average pax. I've noticed -- and am still amused by -- how much the mic actually picks up and the conversations you can hear from across the galley when someone is holding the talk butto ndown.... I had never noticed those with disposable earbuds or over the aircraft speakers (and I know if FAs knew I heard some of the things I've heard they'd be more careful about holding the talk button down when they aren't actually speaking for public consumption )