I was looking at the settings found in Keyboard -> Keyboard Shortcuts -> Sound and Media, I noticed that, while there was an option for Volume up and Volume down, there wasn’t any settings like this for microphone adjustment (aside from toggling mute). Does there happen to be a reason this doesn’t exist? I could make a custom shortcut with something like amixer set Capture 10%+ (in my case), but it doesn’t have the pop-up bar showing the adjustment like Volume up does (I like pretty things 
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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 
This makes sense. If no one else replies today with more information, I’ll go ahead and fill that out.