AirPods volume suddenly too low in Android mobile device
For future reference, the solution that works for me every time is resetting the AirPods by pressing the button on the back of the AirPods for 15 seconds.
Then I will need to pair them again and it will work fine.
This solution can be found here and here.
Note that this is only a temporary fix. The volume issue will continue to appear randomly and this solution will have to be applied again and again...
If anybody has a solution that fixes the issue forever, please share it.