For others wondering about this, here is what the manufacturer says. I am packing to return mine as the volume is VERY low, even when source is at max.

Will I need an adapter if the airplane has two-prong (or three-prong) audio jacks

May 30, 2023 13:45

If the airplane you're on has two-prong audio jacks, we recommend an "airline flight adapter" for use with your AirFly (removed link). This adapter has two prongs that plug into the airplane’s in-flight entertainment system and a jack for headphones into which you can plug the AirFly.

On an airplane with a three-port jack, you can use a 2-pin adapter and ignore the third (smaller) prong. The extra prong is used for special features such as noise cancellation on airline-supplied headphones but is not needed when using the AirFly with your Bluetooth headphones.

NOTE that the AirFly will work without an adapter in two-prong or three-prong audio jacks, but you will likely only get audio in one ear. For stereo, you'll need that second prong.