15 August 2019 at 5:57 pm
So knowing a little bit about Ford's audio system architecture (all Sync versions are quite identical in this area from the original v1 to Sync 3) and some of their existing 'quirks', I may have some pointers that may lead you in the right direction.
Essentially the way Ford has the system laid out baseline is you have the APIM which is just the Sync module and audio-wise just handles Bluetooth (calls and music), Navigation audio, USB playback, and voice prompts and tones.
They also have the ACM which is basically a stripped down old school AM/FM/CD headunit with no display. This obviously handles what you'd expect as I already mentioned: AM/FM/CD/Satellite Radio and most importantly amplifies and outputs the audio to the speakers. It also has extra duties in the form of mixing audio. It will take audio from the APIM (on separate lines, it'll take full audio from bluetooth music playback and USB audio for example and also a separate line for voice prompts and tones) as well as commands over the CAN bus for various functions.
So say for example you are listening to FM radio and have navigation operating. The ACM will happily play its own source right out to the speakers. As you are driving the navigation needs to read off a turn for you. APIM feeds that to the ACM and tells the ACM "duck the FM audio and mix this voice prompt on top". This is also how the same volume controls will handle the different sources depending on what is active as that passes through the APIM first which determines how it commands the ACM to do its part.
What has been common especially on older MyFord Touch vehicles in a similar vein is randomly the ACM will get 'stuck' and not take any further commands from the APIM. So whatever the last audio source was is where it'll stay and switching inputs on the APIM will do nothing, volume won't change, and voice prompts and tones won't come through (if you happen to have parking sensors, those beeps also fail to come through as a fun FYI!). The most common fix is classic IT 'turn it off and back on again' but by pulling the fuses for the audio system. It fixes it for a time.
Based on that information I am taking a wild guess is something is amiss with the ACM and it may be stuck on a silent AM/FM station hence the static and the volume control is out of whack. If you happen to have a premium audio system with a separate amplifier it may also explain why there's the significant jump from 0 to 1 volume (0 would just tell it to cut the audio altogether and 1 is coming back on at whatever arbitrary volume it 'thinks' it is being told to be at). So while I can't make any further guesses beyond that, maybe that background info might help.