Not sure if you would be comfortable with this idea, but you could download nircmd and use something like system/exec/backticks to call nircmd.exe to mute/unmute your system volume.

The syntax of nircmd to mute the system volume is nircmd.exe mutesysvolume 1 and use nircmd.exe mutesysvolume 0 to unmute the system volume. On my system running Windows 7 (64-bit), I was able to use both the 32-bit and 64-bit versions of nircmd.exe to mute/unmute the system volume.