Hi
NikolajThank you for your asking. I just tested my device with Airpods and ASUS cable earphones in the latest firmware version WW-16.2018.2007.68 on Youtube, yet I didn't encounter the same issue. I listened the music on ZenFone Max M2 in 50% volume and iPhone in 40% volume , the two volume performances are already really loud for me. Kindly provide the information below so that we can better assist you.
1. Do you encounter the volume issue in calling, music playing, gaming, or overall circumstances?
2. Please try to enter "Safe Mode". If the function works well in Safe Mode yet the issue remains out of Safe Mode, please check on any disturbance from the installed 3rd party apps.
# How to enter and exit "Safe Mode"
https://www.asus.com/support/FAQ/1005163/3. Delete the connection between Airpods and Max M2; re-connect the two devices again.
4. Also, please try "the Factory Reset"
Settings>System>Reset options>Erase all data(factory reset)
Please note that once the Factory Reset is completed, all data, apps, settings and personal information will be deleted.