Regarding this, please try the followings and see whether the issue would be resolved:
1. Make sure the volume of your phone itself is turned up and on the system settings of your phone, the notification sound of the EZVIZ app is allowed.
2. Make sure the Message Notification Tone on the EZVIZ app (Library page > Tap the second button on the top right corner > Push Notification Settings > Notification Sound) is not muted.
3. Make sure your phone is not in Do Not Disturb or Focus mode or Silent mode.
4. Turn off battery optimization as battery saver mode may limit some activities to conserve battery life and thus causing this issue.