i think that this might have something to do with the vent on the balcony of the building with double tap. Maybe the radio is in there? I mean, it does echo, and i cant remember seeing any megaphones or anything around the place. If you do find something, i told you !!!
btw - the mainframe is a good idea, but there is only one known walkie talkie there, and i dare say that was the first place everybody looked around for radios and stuff. Another idea - maybe its that big cage to the right of the PAP (if you are facing away from it). Maybe you need to turn the power on first...
lol - i dunno. just guessing, but hey, if you find anything, i told you !!