You can set up custom keyboard shortcut to do this:
- Go to Settings > Keyboard > Shortcuts > Custom Short Shortcuts
Here add new shortcut.Give a name to it and for volume up give the command as:
pactl set-sink-volume 0 +2%
For volume down ;
pactl set-sink-volume 0 -- -2%
Now set the required keyboard short cut by clicking the entry and then pressing the required shortcut.
Once done, close it and test. This solution should work since the we keep the n% change small and by continuous pressing for longer time the n% adds up to give bigger change.
You can also set fixed volume shortcuts like 25%, 50% , 75% and so on. Just use,
pactl set-sink-volume 0 n%
n to the required number.