This probably isn't a particularly hard idea.
The thought is to create 2 new commands:
volumeup
volumedown
Volumeup increases volume in increments of .1 up, from 0 to .1 like the cvar.
Volumedown decreases volume in increments of .1
When either command is used, it briefly displays block quake characters on-screen indicating what the new volume level is (character 139 for filled, character 11 for empty or whatever, that's just an example).
And maybe play pak0.pak sound sound\doors\airdoor2.wav each time so the user can tell if the volume level is right.
I have always felt the view size adjustments bound by default to "-" and "+" keys are almost never used. I'd much rather have volume control bound to those keys.
Many players using headphones or speakers without volumne control dials have no other convenient way to adjust to volume while playing.
Just another small improvement idea for JoeQuake.