I believe that many people like to listen to music when running. Recently, I encountered a problem that when my Android phone is connected to AirPods, the phone is turned to the loudest but still very quiet, which also caused me to suspend my running plan for a few days. 
After several days of exploration, I found a fix, let’s share this method today!
1. Connect the Android phone to AirPods, turn the volume to maximum, and open the phone settings at the same time, find: MIUI version, tap seven consecutive times to enter the developer option mode. 
2. Enter the developer options, find and turn off the absolute volume, as shown in the figure: 
3. Open the Bluetooth settings, turn off the media audio, and then turn on the media audio. You will find that half of the volume is displayed, but the volume is exactly the same as the one just turned on same size. 
4. Friendly reminder: Remember to quickly remove your Bluetooth headset, otherwise the volume suddenly increases after you fix it and you may not be able to stand it. . .
Finally, if there is still a problem, it may be a hardware problem with the AirPods. It may be that you have fallen or the earphones have been exposed to water. You may have to consider replacing them.