Why can't I hear anything during a doorbell call?

There are several reasons why you may be unable to hear audio during a doorbell call.

Verify the Alarm.com Alarm.com app is updated to the most recent version.
Verify the media volume on the mobile device isn't muted and the sound is turned up.
Verify the app has access to the mobile device's microphone.
Verify the mobile device does not have any Do Not Disturb settings enabled.
Listen for sound out of the device's speaker as well as with headphones to determine if the issue to related to one of these.
Test other applications for audio functionality.
Test a doorbell call using another mobile device.

For device-specific instructions about how to verify these settings, refer to Apple Support or Android Help.