You can buy the pro version of Sizzling Keys. It is a preference pane
that allows you to define a custom keyboard shortcut for modifying system volume, among lots of other things.
Alternatively, you can modify system volume using AppleScript.
Open up AppleScript Editor and enter
set volume output volume 100
Volume is on a scale of 0 to 100. You can either set an absolute value (e.g. 100 for full volume), or create scripts that increase/decrease, such as the following:
set vol to output volume of (get volume settings)
if vol > 90 then # 100 max
set volume output volume 100
else
set volume output volume (vol + 10)
end if
For Volume Down:
set vol to output volume of (get volume settings)
if vol < 10 then # 0 is min
set volume output volume 0
else
set volume output volume (vol - 10)
end if
If you'd like to replicate the feedback sound that normally occurs when you change volume, you could add the following to the script:
do shell script "afplay /System/Library/Sounds/Pop.aiff"
You can save scripts as these as applications, or integrate them into the Services menu using Automator as an input-less service. You can define keyboard shortcuts for services in System Preferences » Keyboard » Keyboard Shortcuts » Services
1After much trial, I found out that key assignments work outside Finder only if you set
Full Keyboard Access
toAll controls
– Tolga Ozses – 2016-05-12T10:38:08.8333This was perfect. For those who haven't done it before: run Automator, File/New/Service, under Service receives selected choose "no input", on the left next to Variables search for "Run AppleScript" and double-click it, paste one of these scripts below where it says "your script goes here", click green run button to test it, save as "Volume Up" or "Volume Down", System Prefs/Keyboard/Shortcuts, select "Services" on the left, these should show up at the bottom where you can assign keyboard shortcuts. I had to run the services manually from the services menu once before the shortcuts worked. – nonagon – 2014-06-13T18:44:38.143