Like many of you, my notifications volumes was minuscule compared to the ringer volume on my iPhone 13 Pro running iOS 15.0.2.
Then I found the following solution suggested (I don't remember where I found it):
Settings -> Sounds and Haptics -> Ringers and Alerts
Drag the volume slider to minimum, then drag it to maximum, then back to your desired volume.
That's it; the volumes of notifications are now well-matched to the ringer volume.
milesz1
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