Q: Audio on windows 8 wont work

On an iMac which is older than Late 2013 models, using the EFI method does not work. If you want a detailed explanation, I can provide it, but it is full of jargon and acronyms. The best method is to wipe the current installation after you save data (not applications that you need) and re-install using BC Assistant (only).

What year/model is your iMac?

Can you boot into OSX Recovery - OS X: About OS X Recovery - Apple Support? First try Command+R when powering up your Mac which will bring up Local Recovery. If that does not work, try Command+Opt+R, which will bring up Internet Recovery.

The Windows key on the keyboard should work as Command key. Once you have Recovery Console up, we can look at your disk partitioning and see what the next steps to recovery would be. Do you have back ups of your OSX and Windows?

Re-installation of OSX from Local Recovery will bring back your current OSX version, and leave your files untouched, provided the underlying disk partitions are intact. Internet Recovery will install the OS that originally came with the Mac, and then you have to upgrade to the current version via the App Store. What is your current OSX version?

Internet Recovery will install ML, while Local Recovery will install Yosemite, provided Local Recovery is intact.

In Recovery console, if you want to post your current disk partition, in Utilities -> Terminal, post the output of the following commands.

gpt -vv -r show /dev/disk0

fdisk /dev/disk0