Set "Options > General > Advanced > Media key mode" to Disabled. Also try "App commands" and "Hot keys (ignore volume)", because it is the combinations of settings in MC and Windows that effect how the mouse buttons work. Test each setting with MC in focus, and not in focus. See when it controls volume on your amplified DAC, and when it doesn't. Choose the best combinations of settings for you.
Naturally you got control back when you set MC to use System Volume, because your mouse is controlling System Volume, which MC is capturing and adjusting its volume, which is linked to the Windows System Volume.
Naturally MC26 still has control of of the volume even when it isn't in focus, because you set MC to control the volume, through the Media Keys setting.
Use Internal Volume in MC. If you want your mouse to control the Windows System Volume, fine. Disable the Media key mode. MC will only control volume when in focus and using the usual functions inside MC. i.e. The slider and the mouse scroll wheel, when hovering in the correct spot.
Media Key controls in Windows are just a little bit dodgy and unreliable, I find, for some hardware. Or if they have their own drivers. JRiver can only do what Windows allows.
Finally, use the
Release Notes. They have some interesting stuff in there, such as:
26.0.67 (4/22/2020)
2. NEW: Made mouse wheel volume over the player optional.
26.0.7 (11/21/2019)
18. NEW: Added the media key mode "Hot keys (ignore volume)" to leave volume commands for the operating system.