Left speaker on headphones is muffled

hey , im having trouble with my old sony DR-6A headphones from the 60s. they've been pretty smooth for the longest time, but recently the earphone on the left side has been getting more and more quiet.

recently the fell off of my desk, and the left side of the headphones basically went mute, you can still hear it somewhat but it's way too quiet. how do i open these old type of headphones? they don't have screws, just these little rivet thingies.

any help would be appreciated.