The black diamonds, as far as I know, are buildings which are scattered around the place. 'Fraid I don't know if he would be able to see it. That's our good ol' narrators job.
Can it just be assumed Rahj and Shawn listened to their tapes at the same time but couldn't the other's over the sound of their own? I don't want to make a post with a bunch of fluff for "Presses Play" ya know :p
Whichever is more convenient for you O Mighty Beowulf :p If you'd rather wait and see if Demensia presses play likewise, you can just box them all up in a neat little bow.