The problem cannot be fixed with any of the suggestions, I have tried around 5 of the recent suggestions. The problem appears to be connected with, but not limited to, connecting the iPhone to your auto audio by bluetooth. When you connect to your auto by Bluetooth, it probably has the volume set to around 75% in some cases, but in your car the audio will be raised to 75% volume, and that’s quite high. What do you do? You turn the volume most of the way down. That’s where the volume level will likely be. This likely will also occur when you connect to an earbud, or headphone and the results will end up the same.
A new scenario happened to me the other day, where I was setting at a restaurant table where I (as usual) checked the ringer volume, then the audio volume to assure myself I can hear the “ringer”. One instance the volume was at around 10%, so I raised the volume with the side button, and it dropped to the bottom. This occurred several times when I was setting there. I shut the iPhone down, and restarted, then set the side button volume to around 75%, and it stayed there.
Apple must re-write their software to separate the volume control to a scenario that is separate between a car audio device and a headphone. We don’t need to blowout our eardrums, yet when the ringer rings, we must be able to hear it.