Did you ever figure this out? Have 2 clients with exact same setup (8ch NVR, 1 doorbell, stand-alone and configured via wifi to NVR, and 7 hard wired cameras) and just one client with this issue
Click on the doorbell then click on the three dots in the top right corner. Go to Settings, Notification, then change Notification Sound Mode to Mute. Click the Save icon on the top right.