@father_of_6 yes, that seems to be the problem. The same thing is happening with my wife’s iPhone 13 Pro Max since the xmas update.
As
@freeAgent and
@Randino mentioned, you can lower the Ringer and Alerts volume as a band-aid to save your eardrums. The issue appears to be that the Ringer/Alert volume is set higher than your current music sound level, the phone receives a notification or is locked/unlocked, the volume of the stereo is then set to the Ringer/Alert volume (which may be higher than your music volume up to 100%), and then your music becomes deafening.
@adayley had a good post quite some time ago in this thread: