Bootcamp installation of Windows 8.1 (High Sierra 10.13.6) audio driver doesn't work (iMac Mid 2011)
In 2012, Apple starting switching over from DVD installs of Windows to flash drives. In 2015, Apple again switched to installs of Windows where only the Windows ISO file was required. Your have a 2011 Mac and therefore were only suppose to install up to 64 bit Windows 8.1 by using a DVD. While by 2011 the industry had converted from legacy BIOS booting of Windows to the modern EFI boot method, Apple did start converting until 2012. When a High Sierra was installed on your Mac, the firmware was upgraded to allow EFI booting of a Windows 10 installer from FAT32 and ExFAT formatted flash drives.

Unfortunately, the Boot Camp Support Software for your model Mac contains drivers for the legacy BIOS booting of Windows. Therefore, if you attempt to install an EFI booting Windows you may encounter problems such as the audio not working properly. The same should also be true when installing Windows 8.1


  No Mac, regardless of model year, can BIOS boot from a flash drive.


In my opinion, Windows 8 and 8.1 were terrible versions of Windows. Therefore, you should consider installing Windows 10. Since the Window Support Software drivers are designed for a BIOS booting Windows, you should only install Windows to BIOS boot.

If you use a DVD to install Windows, then you should choose to BIOS boot from the DVD. This will automatically select Windows to be installed to BIOS boot. If installing the current version of Windows 10, then you will need a double layer DVD (DL).

If you choose to use a flash drive, then you can only EFI boot from the flash drive. The Windows installer will by default require you to install Windows to EFI boot. Instructions on how to install Window 10 to BIOS boot are given in the accepted answer to the question How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?