AirPod volume low

Did you set a volume limitation on your iPhone? Check if you did by visiting this page here: Adjust the volume on iPhone. And turn it off by reversing the steps mentioned. This will probably allow you to play music through your AirPods at a higher volume.

Hmmm, that does sound weird. If you cleaned them properly, it means that this shouldn't be a hardware problem of anything physically blocking the sound vents. And if there is no setting on your device that is limiting the volume, then this should be a non-hardware problem of the AirPods themselves.

Try resetting your AirPods. Resetting them can potentially fix some certain problems.

Sometimes, firmware updates may make minor changes to certain aspects of a pair of AirPods, so make sure all of your AirPods are up to date. Here are the steps to check and update the firmware of your AirPods: How to update the firmware on your AirPods (note: this is an external link to iMore, Apple Support doesn't have an official article about this). The newest update for 2nd-generation AirPods is 3A283, which introduces Automatic Device Switching. I don't know about 1st-gen. though, since I don't own one anymore.

Also, do all three sets have the same volume, or is it one set that has significantly less volume?

blow into the speaker of rub it with swab and rubbing achol