... you install RealTek driver manually yourself. As for the 'unsupported' then I assume you have 2008 iMac or something early?
Yes, blocking installing a device driver is strange. And because you don't have 2011 iMac just use 3.0-3.3 instead.
BCA in Lion should download Boot Camp 4.01+ set.
So the Bootcamp 4.0 drivers won't work with my 2008 iMac? I'm running 3.3 then, and the problem persists.
I've just tried uninstalling the RealTek drivers; Windows seems to auto-install a "High Definition Audio Device" that DOES play sounds through the headphones... albeit incredibly quietly. Barely perceptible at full volume. I've tried to install the Cirrus drivers from the 4.0 Bootcamp disk; they claim to install correctly (in less than a second, which makes me skeptical), but they still don't appear.
It seems I can have Realtek and no headphones, or no Realtek and minimal sound...
Fixed the problem, after giving myself a sodding headache trawling through 3 years of "solutions" for Macbook Pros. It seems the RealTek drivers included in the Bootcamp disk (that I had to install in compatability mode) don't quite cut the mustard.
Get the drivers from the Realtek website directly, the 64-bit install works without using troubleshoot side-steps. Make sure you don't have any other audio drivers enabled in Device Manager (ie. High Definition Audio Device), reboot your system, spill coffee on your lap when your headphones suddenly start working very loudly.