I don't recall this being reported before, and there are no bug reports about it, that I can find.
Do the 4 systems you played the games on share a multimedia keyboard brand or laptop manufacturer?
What are the brand(s) and model(s) of the keyboards/laptops? (that you know or still have access to to check)
I assume if you were using the same hotkey software, etc, on all systems you would have tried exiting out of that, but you can try doing a clean boot and then test the game, in case a multimedia program, or something, is part of the problem.
Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, Click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.
If you use a program like KeyCodes
, what codes are produced for the volume down and mute buttons? The first keeps a scrolling list of keystrokes, so may be better for troubleshooting (comparing to other keys, etc), while the second provides more detailed information.
With my Roccat Isku keyboard, the volume down key has a key code of 174, KeyData 16777217; mute is 173 and 16777217 (so the same KeyData value). I don't know how much standardization there is with multimedia keyboards, but with the generic multimedia keyboard on my very old PC, the key codes are the same for the volume down and mute buttons, while the KeyData values are different (and unique).
It is possible the keyboard is using codes outside of what is normal, so those keys correspond to hard coded functions (or at least produce consitent results in the way the codes are handled). A background process seems more likely, with this happening on multiple systems for you, but not reported by anyone else (that I recall).