I've found a REALLY cool combination of hardware and software.
I have a Behringer XTouch Mini. It's a USB based MIDI controller with 8 rotary encoders (like a volume knob version of a mouse wheel, infinite turn with clicking and a button when pressed down), 1 analog fader and 18 buttons (besides the 8 buttons that are part of the encoders).
I don't do music stuff, I bought it to try MIDI programming to use things like this as game input devices. Nice little device, and pretty cheap (about $125au). Anyway, this post isn't specifically about the XTouch Mini.
But a week ago I discovered a free program called MIDI Mixer. It lets you bind MIDI devices to various windows functions. The developer even uses an XTouch Mini for testing, although any MIDI device should work, it's very configurable.
You can bind any rotary encoder or fader to different audio outputs to control volume. I currently have Firefox on knob 1, X4 Foundation on knob 2 and Spotify on knob 3. This means while I'm playing a game I can control the volume of youtube videos playing on my second monitor without affecting the game. By default there's a row of mute buttons, one for each knob.
I've also got the fader set to control the master headphone volume and 4 buttons set to media play, media stop, media prev, media next (rather than doing a function shift thing on my keyboard).
You can also bind buttons to change audio input and output devices, such as selecting Oculus output or speaker output, etc. They can also be bound to running files or sending key combos.
Finally, the default action of the knob button press is to bind that knob to the current in-focus program's output volume, so you don't need to configure new programs in the UI, just start some sound playing in an app/game and press a knob to get volume control.
Midi Mixer: https://www.midi-mixer.com/
