Ok, I understand I may be a bit late to the party, but I had just solved the same problem with Windows 10. Now I am writing this under my Windows 10 account, while listening to Aristocrats Radio =)
I have a MacBook Pro 13" (mid-2012), 500 GB SSD and 16 GB RAM.
This issue boils down to a poor EFI support on older MacBooks. Boot Camp assumes it can do just fine with this, when creating a USB installer and partitioning the hard drive. However, EFI boot mode exposes devices in a different way, which effectively leads to audio not working.
Basically, the solution is to use a legacy MBR boot mode, when installing Windows. However, due to Boot Camp's tricky partitioning of a hard drive, legacy mode will not allow Windows installation to that partition (the infamous message "Windows cannot be installed. The selected disk is of GPT partition style" etc.).
To install Windows 10 without any of these problems, I did the following:
0. Saved the latest Windows drivers to a USB stick via Boot Camp.
1. Burned a Win10 ISO image to a DVD.
2. Merged a Boot Camp-created partition back to a single MacOS partition (did that via Boot Camp).
3. Created a new partition for Windows 10 in a Disk Utility (I assigned 100 GB, formatted to FAT).
4. Re-formatted this new partition to NTFS, using a third-party software. I used Tuxera NTFS for Mac (http://www.tuxera.com/products/tuxera-ntfs-for-mac/). It has a 15-day free trial.
5. Put my fresh Win10 DVD to an optical drive and rebooted, while holding Option key (alt).
6. In a boot options screen, I selected DVD "Windows" (Do not choose EFI boot.). Windows installation started.
7. In a Windows Installer Disk selector screen, I chose to install Windows to that NTFS partition created earlier.
It all went smoothly afterwards. Windows reboots a few times during installation process, as usual. In these cases please hold Option key during reboot, and then select to boot from hard drive "Windows".
After Windows is installed, do not forget to install latest Windows drivers from your USB stick.
NB: Before all that, I also played a bit with marking my MBR partition from hybrid to protected (see here: http://fgimian.github.io/blog/2016/03/12/installing-windows-10-on-a-mac-without- bootcamp/). However, I am not sure this actually was important in my case.
