Been reading the different replies to one airpod louder than another, or one not as loud as the other, in this instance there seems to be more left AirPods with reduced volume to the right. Yes, in accessibility in settings you can adjust the balance, but that doesn’t fix the problem. That merely lowers the volume of the right side. Then I read a number of posts about sucking the affected lower volume AirPod. To my amazement the left side was difficult to get any air to pass through, initially. I tried the right, and completely free flowing air. I kept at the left side and finally air started to flow. Kept at it for a while until they were almost equal, but still a slight resistance on that left AirPod. However I connected them and listened and they were now almost equal in Volume. Something, probably dried earwax, was/is blocking a port, probably a bass reflects port, which would keep the small diaphragm of moving as easily.
Try the lower volume AirPod and compare it to the other side when sucking on it and you will immediately notice a difference if it’s the same problem. Best, Seth