When you update your iPhone to iOS 14 and suddenly your iPhone showing headphone audio levels warnings on your iPhone then you need to turn off headphone notifications and reduce loud sounds and then you need to restart your iPhone and check for any software update available and update your iPhone to fix headphone audio levels warning on your iPhone.
Update your iPhone to latest version of iOS may solve this headphone notifications issue.
Launch settings on your iPhone -> tap on Sounds & haptics -> tap on reduce loud soundsĀ and then turn on reduce loud sounds, Now you need to use slider and drag the slider to the maximum decibel level for your headphone in which you are comfortable hearing music.
This is a temporary software glitch of iOS 14 update by just restarting your iPhone you will get rid of this headphone warning on your iPhone after updating your iPhone.
Reduce loud sounds and turn off headphone notifications, restart your iPhone and update your iPhone if updates are available.
Settings->Sounds and Haptics->Reduce and Sounds -> turn on reduce loud sounds -> drag slider and set volume settings here.