ok I may have found a clue to what might be triggering this... I think it has something to do with Atari easter eggs. Here is what I've observed.
I was thinking about the problem and I remember after I got the 2019.12.1.1 update, I went through the release notes to see what was new - e.g. I quickly enabled dog mode, and opened up the Atari easter eggs. I started a game of Breakout but only played a few seconds because the audio volume was kind of loud. Then I exited the easter eggs and didn't return to them again. It was after that, later when I went for a drive that I noticed that shifting gears mucked about with the audio player volume. As reported up-thread, originally it was changing volume to 2 every time I shifted gears, later it started changing volume to 4. Then I rebooted the MCU and the problem seemed to go away.
Today when I remembered the easter eggs, I opened up the Atari games and launched Breakout. I only played a couple seconds, long enough to hear the loud game sounds, then I exited the easter eggs. Later when I went for a drive the audio problem started again! i.e. shifting gears affected the audio player volume, this time (again), forcing the volume to 2 every time I shifted to D,R,N (but not P)
so I'm guessing anybody else seeing this audio weirdness likely launched one of the Atari games beforehand. Somehow playing an Atari game mucks up something in the logic and causes shifting gears to modify the audio player volume unepectedly. Chalk another one up to lame programming skills at Tesla.
No idea why I was seeing volume -> 2 and volume -> 4 on different days. After this latest confirmation of the problem, I've held off rebooting to see if something changes after a day (like it did the first time)
A lot of people (me included) have been complaining how Tesla is wasting engineering resources (and MCU memory resources) on easter eggs when it'd be nice if they put even the smallest effort into fixing 3 or 4 year old audio player bugs. Well here seems to be a case where the easter eggs are directly having a negative effect on the audio player