Jochen is probably stuffing his face with cookies right now (inside joke).

MUTE is indeed active low and TELON active high, but they are only for the phone. The nav computer does not have a connection to TELON on the harness.

Looking at the wiring diagram, it seems the TV Tuner is also using NAVNF for audio, would I be right in sayng the TV can create audio while you browse the menus? In that case maybe it does work on detecting audio, as NAVBUS does seem to bypass the video module.