For now Windows has but one audio mixer so my bet is you mastered that. So what to do next is to see if there's a way to push up/down volume in the game, then in the Windows mixer up/down until you find the right mix setting.
What you noted is true. That is, for now a limitation of Windows.
I'm having trouble with my volume while running two different programs. I'd have a game open on one screen while having YouTube or Pandora open on a second screen. When I switch to the game, the volume decreases on my Google Chrome Browser. I've looked online for answers and the same answer came up, to go into audio devices > communications > select Do nothing. And everyone says this works. However, It's not working for me.
In Device Manager in Sound, video and game controllers I have
NVIDIA High Definition Audio
NVIDIA Virtual Audio Device (Wave Extensible) (WDM)
Realtek High Definition Audio.
My motherboard is a
MSI Z87-G45 gaming
I've looked all over Google and found nothing which surprises me, I know I can't be the only person with this issue. If someone could help me fix this i'd really appreciate it.