Hey fong47,
If we understand correctly, the notifications sound volume is less than before you updated to iOS 17. There is no way to revert to a previous version of iOS. If you use a different notification sound, is it still low?
"You can choose the sound volume for alerts, choose between a tone or sound for alerts, or change the notification vibration pattern (when the phone is silenced).
- Go to Settings
> Sounds & Haptics. - Do any of the following:
- Make notifications quieter or louder: Adjust the volume with the slider.
- Choose a tone or sound: Tap Text Tone, then tap a sound below Alert Tones (or tap Tone Store to download one).
- Choose a vibration pattern: Tap Text Tone, tap Vibration, then choose an option (or tap Create New Vibration to make your own).
- Tip: The tone and vibration you choose apply to both the Messages app and other apps that have notifications on, such as Mail, Phone, and other Apple or third-party apps. To change them just for the Messages app, go to Settings > Notifications > Messages, then tap Sounds.
You can set how many times to repeat a notification (from Never to 10 Times) in Settings > Notifications > Messages > Customize Notifications."
Change message notifications on iPhone - Apple Support
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