One of the most common reasons why one AirPod may sound louder than the other is the accumulation of dirt, dust, or earwax on the speaker grills. To clean your AirPods, you can use a soft, dry, lint-free cloth to wipe the surface of each AirPod. You can also use a dry cotton swab to reach the hard-to-reach areas, such as the small cracks and crevices.
For the speaker grills, you can use a soft-bristle brush or a toothbrush to gently brush away any debris that may be stuck on them. Do not use any liquid or sharp objects to clean your AirPods, as this may damage them or cause water damage. After cleaning your AirPods, make sure they are completely dry before placing them in the charging case or using them again. This way, you can ensure that your AirPods sound balanced and clear.
I might say this is the best method to fix this issue, However there are other fixes too, but I can't elaborate each one, its best you search on google and find a relevant guide or you can visit this guide that has provided 8 Solutions on how to fix volume on one AirPod lounder then other