A quick comparison of the keyboard volume between a iPhone 7 Plus and a iPhone 6s Plus. Both iPhone have max volume and are both running iOS 10.0.1. The volume on the iPhone 7 is half that of the iPhone 6s making it almost impossible to hear.

The iPhone 7 is on the left the iPhone 6s is on the right.

It isn’t just the keyboard volume that is low other system sounds on the iPhone 7 are much lower compared to a iPhone 6s.