@BearBu FYI I finally installed my HDMI box hahaha.. only took more than a year...so for anyone having trouble with audio, here's a tip (P.S.. I'm on MCU1 AP1...Apparently the wiring hasn't changed up until Raven)..
But, at the back of the MCU, there's a brown connector harness on the middle row. The right-hand one if you're looking at the back of the MCU with the screen facing away from you. That harness is the radio tuner "analog" input... This is important because you can directly feed stereo L/R signal into here... What's more interesting, is that the MCU (or radio), seems to detect this incoming signal somehow. For example, I have the radio sitting on a known FM station.. it's playing the station perfectly fine. I've piggybacked a 3.5mm stereo socket to this harness and plugged the socket into my phone. As soon as I start playing anything on the phone, the FM radio audio completely disappears and the phone music takes over. As soon as I stop playing the music, the FM radio resumes.
This is why it took me a year to install... because it required splicing into factory MCU harness... and I wasn't going to do that whilst the car was still under warranty.. That warranty lapsed... so... the mods begins!
The harness should mark out pin numbers too for anyone else interested.. The pinout assignments are as follows:
L+: Pin 10 (Grey wire)
L-: Pin 20 (Brown wire)
R+: Pin 8 (Light Blue w/ Black Stripe)
R-: Pin 18 (Dark Blue)
So
@BearBu that begs the next question... Is there an audio HDMI decoder inside the interface box? Are there analog audio L/R signals inside the box that can be accessed? If so, that would make it the best solution because it means someone merely needs to switch to Radio mode, irrespective of if they're on a channel or not. And of course when the hdmi input isnt playing anything, the radio resumes.
It gets even funkier for those of us with the Premium Audio option.... HDMI 5.1 decoding anyone?
... for now I'm sufficiently happy with stereo output....