The lack of volume support is intentional - because otherwise you would get widely inconsistent volume between MC playing media ("input" is always full volume), or media being played through the WDM driver ("input" might not be full volume anymore).
The volume should be controlled on the actual output device, not the WDM intermediate stage.
What works for most people is simply using the media keys on your keyboard, for example, as MC will receive input from those and then reduce volume of the output, instead of doing it on the input.
That said, no games I ever played have certainly messed with the system volume, as that would not only affect them, but other apps as well, which would certainly not be wanted. In fact most games let you change volume of music, speech, effects etc independently, which certainly wouldn't be representable through system volume.
Thanks. The explanation makes sense. Indeed you are right about most games having their own mixer for music/speech/effects. I thought some apps might open the device exclusively, and then set device volume, but perhaps not.
I still wish the Windows volume mixer would work on the overall device, though. At least as an option. It's very disconcerting that it doesn't. The fact that people need workarounds with media keys going to MC should tell you there might be some demand for the feature still. Volume mixer does work on an individual app, but I often have a large number of apps, and finding the one that's sounding can be annoying. I am just used to reducing it at the device level in the mixer.
Here is an example of odd behavior in Volume mixer that's cause by the lack of this feature.
Start with all apps closed. Volume mixer will show just the JRiver device, and "system sounds". Start with them both at 100%.
Now, drag the JRiver volume down to 50%. Windows also drags "system sounds" level in the GUI to 50%. But everything still sounds full on.
Now, set both back to 100%. Reduce just "system sounds" level to 50%. Now it sounds quieter.
This is the sort of behavior that's just not right, IMO, and that's caused by the driver ignoring intermediate volume levels.
I know I'm not alone in wanting to set volume for all Windows apps, as I have seen many threads on this in my google searches.
At the very least, I hope you consider some improvement for those of us who like to use the mixer, and have it work.
And of course, in Windows 10, the default volume control isn't the mixer, but something that only controls the device volume, and that has zero effect with the WDM driver.
If you really don't want to change the MC output level, then perhaps consider doing an extra software-level volume control for the output of the WDM driver, ie. this would be the "input source" of MC. Seems like it would be an extra unnecessary computation, but perhaps it makes more logical sense ... I'm just throwing things at the wall here, maybe one of them will make sense to you.