The solution that @DrStrangepork mentioned pointed me to the solution. Basically the problem is iTunes (exists on 12.7.1.14, running macOS 10.13.1). I've yet to find the root cause of this problem (either when a phone call comes in via Handoff or some other problem) underlying cause.

What does work however, is quitting iTunes, playing a video on YouTube via Safari. This kicks/resets the media controls, and re-opening iTunes now correctly binds to the media volume.

Hope this helps someone.

What does work is quitting iTunes and playing a video on YouTube via Safari. This kicks/resets the media controls, and re-opening iTunes now correctly binds to the media volume.

The problem is iTunes (exists on 12.7.1.14, running macOS 10.13.1). I've yet to find the root cause of this problem (either when a phone call comes in via Handoff or some other problem) underlying cause.