The solution is to use the freeware nircmd tool and create a desktop shortcut with whatever hotkey you'd like in order to run one of the following commands:
As documented at http://nircmd.nirsoft.net/changeappvolume.html
changeappvolume [Process] [volume level]
Examples:
changeappvolume iexplore.exe -0.2
changeappvolume wmplayer.exe 0.55 Speakers
changeappvolume /1275 -0.25 1
changeappvolume Firefox.exe 0.5