My left AirPod is Quieter than my right AirPod

With a flashlight check and see if your Left AirPod might be clogged with Ear Wax, if so, a large blast of compressed air may clear it out or as so have found blowing with force can sometimes also clear.

Try a reset of the AirPods and see if that works also.

How to Factory Reset AirPods

1. Put your AirPods in their case—keep the lid open.
2. Hold the button on the back, as shown above, for three to five seconds until the amber light blinks three times.
3. Close the lid.
4. On your iPhone, go to Settings > Bluetooth > AirPods and Forget This Device, as shown below.

Sorry, do not blow air into AirPod - deggie is correct, take a look at this as well from others who have had similar issues.

Sep 21, 2017 2:28 AM in response to Marie-Claude Leblanc

I know this sounds unlikely, but give this a go. I can tentatively confirm it works. My right bud was considerably quieter than the left. I sucked until I could feel air flowing through the smaller speaker, and sure enough, they had the same volume. Thankfully I didn’t inhale any wax. 😁

Apr 5, 2017 3:53 PM in response to paulosoaresf

It may help to check your settings:

Go to: Settings > General > Accessibility > under "Hearing", check and, if required, adjust the audio volume balance slider between the left and right channels.

NO, do not recommend blowing air INTO the screen, this will just drive wax farther in. You need to use your mouth to suck out the earwax or use a hearing aid cleaning kit. Your recommendation will damage the AirPods.