1500 XP, Cash
After completing the Message to the Masses quest, talk to Garrett again to begin this one. You will need to activate the rooftop speakers. This one is a little harder because the mini-map only gets you to the building not the speaker.
The first one is pretty easy since the ladder is visible from where the map leads you. There are gun wielding enemies up top.
The second one is a little trickier, you will have to climb the building to the left of where the speakers are. Just stay left and the ladder is near the back of the building. Once up top, walk across the wood bridge and take a ladder down to a fenced in area where the speakers are.
The third one is also easy, just climb back up after activating the second one and stay on that rooftop. There will be a thug guarding it.
The fourth one you will follow the mini-map and climb the scaffolding in the middle of the road, jump to the right to find it. There will be a suicider guarding it.
After that head back to Garrett to claim your reward. Talk to Garrett again to start The Second Head of Cerberus quest.