I have been dealing with this issue for the past couple of months and seemed to have exhausted all options when it comes to fixing them. Then I stumbled upon this thread...
At first I was intrigued as to why anyone would think to do something like "sucking on their AirPod" in order to fix a potential hardware issue. I was disgusted, yet intrigued.
I took an alcohol wipe to the left AirPod to ensure that I did not have any built up wax inside of my speaker hole, double checking my work. I then proceeded to "suck" on the speaker portion of the AirPod, sensing a resistance in AirFlow (cwuteyedidthere). After some persistent work, it seemed that the AirFlow began to move freely through the AirPod. After assuming the job was done, I popped the AirPod into my ear to check the sound quality...
To my amazement, the left AirPod's sound was completely restored. My AirPods are back to normal and my faith has been restablished in the product. Alls it took was a little meticulous cleaning, AirPower (cwuteyedidtherev2.), and faith.