Rise of the Crimson Raiders

The first echo recorder can be found on a ledge to the left of the hanging bullets sign, down the alley facing Moxxi's'.

The second can be found inside of Moxxi's, to the right just as you enter.

The third can be found a little ways outside of Marcus' Gun Shop. Face the GUNS sign/entrance to the shop, then turn around. You will see a park bench to your right. walk past that, and the recorder will be on a box on the right sidewalk.

The last recorder is inside Marcus' Shop. Go downstairs to his NPC, the recorder is on the desk in front of him.