Bootcamp audio problem for a Mid 2012 MacBook Pro

I used Bootcamp Assistant to make a USB with Windows 10 and the Bootcamp Support software. ie. 1 USB with both on it !

When I use the 'option' key to boot the Mac with the USB inserted, I am presented with two yellow volumes. One is 'Windows' and the other is 'EFI Boot' (as well as my Mac HDD next to them)

If I select 'EFI Boot' I can make the Windows installation and the support software installs as well but there is NO audio. The Cirrus drivers fail to install and the 'High definition audio' driver has the yellow triangle in device manager.

If I select the 'Windows volume' the install Windows process begins (and looks a bit different) but ends by not being able to install on the MS-DOS fat 32 partition I made, saying '.....cannot install on a GPT disk...." etc.

Obviously I can't make my whole disk MBR because it will delete my Mac OS.

Any ideas would be greatly appreciated. Cheers!