The device in question is a Topping G5, but the issue happens with many USB devices on Windows that have their own inbuilt DAC, but not all. For example my Aune T1 gives me individual channel control just fine.
Normally to adjust audio balance, you open the Properties window for the device, goto the Levels tab, click the Balance button, and adjust the individual channel levels.
For this device in that window, I'm seeing a single slider for the volume, the mute button the right of it. The "Balance" button simply doesn't exist, and there's no other device level control underneath it either. Despite that, sending stereo sound data to it works fine, you can hear left-channel data in the left ear, etc.
It's currently using the default windows-provided driver. I've tried using the Topping driver; no change. Then rolling back, again no change.
I've tried writing my own program, importing the NAudio library and controlling the channels, no luck: it only registers a single channel in windows.
I've tried using the small program "setvol" but the same issue presents itself: it only sees only one channel.
I give this a try, which sounded promising: https://superuser.com/questions/1643029/unable-to-change-audio-balance-on-windows-10 But this is only for one there's visibly dual (left+right) channels and the UI has locked them together, not for when there's only one "channel" controlling the signal that goes to the DAC.
I'm a bit stumped.