Finally I had a chance to look into the Win7 problem.
The short answer is: Run EventGhost in Windows XP sp3 compatibility mode, and it should work in Windows 7. See below for an alternative.
I found an alternative way to get the master volume and mute status:
To show the current master volume: Create a macro, add
Change Master Volume action (from System\Sound card) change it to 0.0% then add a ShowOSD action with
Volume {int(round(eg.result))}%
To show the current Mute status: Create a macro, add "two"
Toggle Mute actions (only one Toggle Mute action will change the toggle status) then add a ShowOSD action with
Mute {("Off", "On")[eg.result]}

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jonib