Maybe this works for others too.
I have added a quick fix for muting on OS X 10.11, but I don't can test this on other versions of OS X.
It seems that there is a difference between AudioHardwareServiceGetPropertyData and AudioObjectGetPropertyData. One shows the capabilities of the Hardware and the other is used to mute or unmute.
Muting works on several audio outputs (internal, blutoothspeaker and airplay).
Maybe this works for others too. I have added a quick fix for muting on OS X 10.11, but I don't can test this on other versions of OS X. It seems that there is a difference between AudioHardwareServiceGetPropertyData and AudioObjectGetPropertyData. One shows the capabilities of the Hardware and the other is used to mute or unmute. Muting works on several audio outputs (internal, blutoothspeaker and airplay).