Hi flamingolee,
Thanks for posting in Apple Support Communities.
We understand that your notifications are low. If you have not done so, restart your iPhone and update to the latest software version to ensure that you have the latest performance fixes installed to address software anomalies.
If you have a case or screen protector on your iPhone, remove them in case they are obscuring the speaker.
Adjust the volume in Control Center
When iPhone is locked or when you’re using an app, you can adjust the volume in Control Center.
Open Control Center, then drag .
You can find this steps and more in Adjust the volume on iPhone - Apple Support.
If this behavior continues, check Settings > Face ID & Passcode. Do you have Attention Aware features turned on? If so, turn it off and test again. Turn Attention Aware features on or off on your iPhone or iPad Pro - Apple Support
"What are Attention Aware features?
Even if you don’t enroll in Face ID, the TrueDepth camera* intelligently activates to support Attention Aware features. With Attention Aware features, iPhone can check whether you’re paying attention to your device and automatically take action. 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, open the Settings app, tap Face ID & Passcode, then turn off Attention Aware Features."
Regards.