I recently upgraded to Windows 10 and now find that certain games (such as Project Reality, the STALKER series and Killing Floor) do not have surround sound - only stereo. It looks as though there must be a driver issue as I never noticed this issue in Windows 7. The odd thing is there are other games (Elite Dangerous and Killing Floor 2) that the surround sound works well with.
Why is it just the older ones and how can I get the surround sound working again?

Oh and I am not using a sound card (just in case that makes a difference).