osxaudosink: missing mute property

Submitted by Thomas Löwe

Audiosinks for Windows and Linux offers beside the "volume" property also a "mute" property.

This is missing for MacOS and so here an additional "volume" element is required to get the option for toggling the mute state.

Would be nice if this could be unified on all operating systems.

Thanks, Thomas

Version: 1.5.90