Rename \Windows\system32\sndvol32.exe to _sndvol32.exe
Cancel Windows file protection dialog.
When you want to change the volume:
Add this to a batch file in \Windows\system32. Then in start menu run just type accessmixer.
accessmixer.bat:
@ECHO OFF
move _sndvol32.exe sndvol32.exe
sndvol32.exe
move sndvol32.exe _sndvol32.exe
You'll have to cancel another WFP dialog when it comes up unfortunately. After it's done executing it will go under disguise again. They can still change volume by keyboard shortcuts though, if those exist. They can also by the sound control panel which you can disable too by using TweakUI.