Does there happen to be a reason this doesn’t exist?
My guess is:
Volume-up, volume-down, volume-mute and mic-mute often exist as special keys on keyboards, and therefore have dedicated keysyms (XF86XK_AudioLowerVolume, XF86XK_AudioMute and XF86XK_AudioRaiseVolume).
The same doesn’t apply to raising/lowering the microphone volume.
It’s probably still worth filing an enhancement request (maybe suggesting <shift>XF86XK_AudioRaiseVolume?). At the very least, you may get a reply from someone who isn’t guessing 