I have a mid 2012 macbook pro with the same audio problem. I spent a few weeks researching and possibly 10-15 installs and full formats before I came up with this installation method and I now have a dual boot system with audio in windows and no issues. I have never replied to these types of forums but this was so frustrating to work out I thought I would try to help others too.
1. Create a USB Installation disk manually as bootcamp fails to create it correctly (just google 'manual Windows USB creation'. Use boot camp to download the macbook drives and bootcamp app for windows. Copy the downloaded folder to the Windows 10 Installer USB. Do not use this USB to setup windows as it will auto load with EFI installation, this will result in failed audio. The USB is needed after installation as the bootcamp app checks for windows files in its directory before loading.
2. Create a Windows 10 installation DVD. This is needed because it will install windows in Legacy mode which means the sound will work.
3. Insert the USB Windows 10 installer. Run Bootcamp (it will check that the windows installer USB is present) untick the first two options leaving the third to install windows. Slide the marker to indicate the size of windows partition you want. Follow the instructions and enter your user password, the first stage creates the Windows partition. (I have previously tried all other ways to manually create the partition but for some reason these manual partitions using ‘disk utility’ and gpdisk’ fail to work).
4. You will be asked to enter your password again for bootcamp to change startup disk. You do not need to enter this. The only thing bootcamp was needed for was to create the partition which it will have already done after entering the first password.
5. Inset your windows 10 dvd and shutdown the computer. Turn it on again holding the option key. Wait for a minute and you should see to pictures of a DVD, select the ‘Windows’ option and not the EFI option which would result in sound not working.
6. The Windows 10 Installer will start loading, follow the instructions, enter your serial if you have one, if not it doesn’t matter. When you come to install options select advanced.
7. Click on the BOOTCAMP partition that had been created and click format. Bootcamp will have created this partition in FAT32 so the windows DVD will need to change it to NFTS.
8. After the format you can now click on the parttion and click ‘next’. Windows will continue installing and just follow the onscreen directions.
9. After everything has setup plug your windows 10 installer USB back into the computer and open the Bootcamp folder then click setup. This will installer all the drivers and the bootcamp app which can be found in the lower right corner of the taskbar
10. Open the bootcamp app and you can setup right click mouse button on trackpad. You can not restart into mac OS from this app. Apparently this option was only designed to work with the apple old HSF+ files system but the new OS are in APFS file system. This does not matter really as you can just use the option key at startup to change OS.
11. By default ater this installation every time you turn your mac on it will boot windows. To changed this hold the option key at starup. Select your desired Drive to start but click it holding the Crtl key. This will change the default startup disk.
12. Another point to mention is that anyone using a third party app like Tusera will not see the windows partion as an option in preferences/starup disk.
13. Also I did not like my windows partition being called bootcamp. I logged into windows, opened file explorer and right clicked the Bootcamp disk then renamed it to Windows HD. This is fine to do and I can confirm it doesn’t mess up your installation in either system.