To test the volume of audio alert go to MORE > SETTINGS and in the PREFERENCES section, select ALERTS. Toggle the switch next to SPEAK ALL ALERTS on and off. This will cause an audio message to play that says "ForeFlight audio alerts are now disabled" or "ForeFlight audio alerts are now active." While the alert is playing, you can adjust the iPad volume as needed.
Image 1. Go to the MORE page and select SETTINGS and ALERTS.
Image 2. Toggle the SPEAK ALL ALERTS switch on and off to hear the alert volume.