iOS 10.0.1problems
SInce i had updated my iPhone5 to iOS 10.0.1 i remarked a drastic volume cut down on my EarPods.
Any restrictions set up for the headphone volume in Settings/restrictions?
Did you already force restart the phone after the update by holding the sleep and home button for about 10sec, until the Apple logo comes back again? You will not lose data, but this can cure some glitches after installing new apps or software.