I installed Windows 8.1 under Boot Camp on my 2012 iMac yesterday, and everything seems to work okay except for audio. The audio mixer shows the audio bar moving, but nothing will play. I've tried with the built-in speakers and through external speakers. I tried using the Audio drivers that Boot Camp provided, but they crashed the audio and I had to go back to the default "High Definition Audio Device" driver.
I'm not even sure what to look for. A newer audio driver?