iPhone Alarm Volume sounds low
Another fix for iPhone X and up could be go to SETTINGS>FACE ID & ATTENTION and turn off ATTENTION AWARE FEATURES. That feature can play a role in lowering your volume for certain alerts. Hope this helps! God Bless!
Question marked as
The volume will only be lowered, if you are looking at the iPhone screen, because the phone caught your attention by sounding the alarm.
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Even if you don’t enroll in Face ID, the TrueDepth camera on iPhone X or later, or iPad Pro 11-inch or iPad Pro 12.9-inch intelligently activates to support attention aware features. For example,
- When you’re looking at your device, your display will not dim until you stop looking at your device.
- If you're looking at your device, it will lower the volume sound of your alerts.
If you don’t want to use these features, go to Settings > Face ID & Passcode and turn off Attention Aware Features.
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copied from: About Attention Aware features on your iPhone X or iPad Pro - Apple Support