Got an iPhone volume issue / problem in iOS 13? It's the side button.
This is a quick post to bring together information on sound issues that is currently spread across a few different threads.
Some users are reporting issues with incorrect notification volume in iOS 13:
https:/https://discussions.apple.com/thread/250642428
https://forums.macrumors.com/threads/i-turn-sound-low-notifications-still-high-please-help.1798024/
Some users (including myself) are reporting issues with incorrect lock sound volume in iOS 13:
https://discussions.apple.com/thread/250764992
https://discussions.apple.com/thread/250667635
https://discussions.apple.com/thread/250642428
It appears that both issues are tied to the side button.
My lock sound volume bug is alleviated (not fixed) by either: (1) going to Settings > Wallet and Apple Pay and disabling 'Double-click side button', or (ii) going to Settings > Siri and Search and disabling 'Press Side Button for Siri'.
Similarly, users with notification volume issues are reporting that both the above actions alleviate their notification volume issues.
Note that these are not permanent 'fixes', as you have to disable some of your phones functionality to restore normal sound behaviour. Additionally, the sound button functionality is re-enabled after software updates, so you'll have to re-disable them after software updates.
Please report the bugs to Apple so these issues finally get some attention.