1) On "Mute", it saves your current system volume and then uses 
Apple's AppleScript API to set it to zero.

2) On "Unmute", it sets your current system volume to the value it 
previously saves and uses Apple's AppleScript API to set it.

If you quit Remote Buddy after you used the Mute action, the level is 
already at zero, so when you start Remote Buddy and then use the Mute 
action again, it won't be able to guess what volume you previously had.

The same applies when you use the mute and unmute key. If the level is 
already at zero when you press it, it will stay at zero no matter how 
often you press it.

To change system volume, press the volume plus and minus buttons on 
your keyboard or use the actions supplied in Remote Buddy for that 
purpose. And unless you quit Remote Buddy since you used the mute 
action, it will also unmute again if you simply execute the unmute 
action again.

Apple does not provide a system-wide Mute/Unmute API, so you'll always 
have to mute and unmute volume using the same method.

Best regards, 
Felix