Interesting (though not surprising if there's inconsistency in behaviour across different cars, we've seen plenty of that). Did you first try playing an easter egg game, e.g. Breakout?
my car is AP1 / MCU1. to reproduce the problem after a reboot, I launched Atari games, played a few seconds of Breakout, then exited after hearing some game sounds. After that, while playing Slacker audio and after shifting the car out of Park, each shift to D/N/R caused audio volume to change. I readjusted volume between shifts to make sure I could hear the difference. Problem also occured while playing USB audio or the radio (presumably also Tunein, I did not try that specifically). Reboot fixes it, until next time easter egg game is played.
Note: I've tried reproducing this a handful of times now and the volume level it changes to varies, it's not always 4 (or 2) - but it definitely changes to one specific level. Actually it gets stranger - a couple times I did this, the Slacker audio stopped playing, as if muted - the progress bar still moving and pause button still visible as if playing - but no sound. I had to change audio source e.g. to radio then back to Slacker to make the sound resume.
bottom line there's definitely some strange bug affecting audio playback if you first launch easter egg Breakout.
