Using AutoHotkey and nircmd makes it easy to use Scroll Lock key to toggle the master volume muting.NirCmd download siteAutoHotkey download site
There is a built in functionality in AHK to send the volume mute key. But it may not work on all systems. More likely to be universal is the nircmd method. If you run the script with Scroll Lock off then the Scroll Lock indicator LED on the keyboard should serve as the mute indicator. This gives another visual cue to go with the system tray speaker icon with the line through it.
Here is the script
Edit: The GetKeyState addition should sync the state of ScrollLock and the volume muting on script startup.
; any other settings here
; sync ScrollLock LED and volume muting on script start
nircmd mutesysvolume 1
nircmd mutesysvolume 0
; Global Hotkey
nircmd mutesysvolume 2
The tilde('~') character stops AHK from "eating" the keystroke. This way the Scroll Lock LED should toggle as you press the key and mute/unmute the volume. To avoid lighting the LED just delete the tilde.
To access nircmd from anywhere on your system it is recommended to copy it to the Windows directory. If that is undesirable then copy the executable to a directory in your Path.