yeah i got this working too...
excellent stuff... i dont want a speaker relay clicking in front of a £700 amplifier hehe.
the mute signal is the opposite of the signal required to switch in the phone audio.
i tried using a relay to invert this singal, but i had problems of it effecting the ignition live wire that i was using to power the relay, it kept holding it up and 12v through the coil of the relay, which kept the stereo on etc when the ignition was on.... kinda hard to explain really.
in the end, i used a transistor 'as a switch' an inverting switch in fact, i think its one transistor with two resitors. then i wired it into the connects 2 box that i used to interface the parrot with the bmw. seems to work like a charm.
it is amazing how difficult this info is to find lol