When I put my AirPods in my ears, I can hear a ping sound, indicating a successful connection to my iPhone. My friend who also has AirPods is sensitive to moderately loud sounds, and this sound is too loud for them. Is there a way to reduce the volume of this sound, or disable it?
No. The system tracks the AirPod volume and the system volume separately so you can keep the phone loud and the AirPods low after they pair, but the connect sound that happens when the AirPods detect they are in your ear canal are set in firmware and not user adjustable.
Unless you can place one in your hand and trigger the connection/proximity sensor this would need a setting to be added to the firmware or software.