While driving, I normally listen to audio books on my phone via BlueTooth. If I finish a book, I'll switch to FM radio - which plays very loud. It's not just momentary, though. I have to reduce the volume a lot (by 8 or 10 clicks on the left hand steering wheel control) before it's comfortable. The next time I try to listen via BlueTooth, I get nothing - until I remember to increase the volume by that amount. Definitely not a good user experience.
I realize the sound level via BT also depends on the volume set on the phone. I have a Samsung Galaxy S7 running Android 8.0.0 (Oreo). If I am not listening via BT, I am usually using earphones, and have the volume set just below the level where it warns about hearing damage. So I would not consider that a low level. Maybe iPhones have really inefficient earphones, so they use much higher volumes, and the M3 expects that high level from any phone. Regardless, I wish Tesla offered a configuration parameter to adjust the relative level of the radio and BlueTooth sources. (I haven't tried Slack.)