You can tell Siri to 'increase volume,' which increases the volume in approximately 12% increments.

Here is how to control the volume of your AirPods: Rather than integrating volume controls into AirPods themselves, Apple has delegated that duty to Siri. First, if your iPhone is nearby, you can simply use the volume controls on your phone - the volume buttons and slider on your phone directly affect the volume level of your AirPods.