Windows 8.1 poor sound quality
The Windows 8.1 and the drivers of bootcamp may interfere with each other. 

As Windows 8.1 is not yet officially released I would say that bootcamp isn't yet fully compatible with Windows 8.1

Though this link might help: http://answers.microsoft.com/en-us/windows/forum/windows8_1_pr-hardware/audio-lost-after-installing-windows-81-preview-on/7033e0ac-ba83-4395-afdf-9a312dbb3d99