On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Torture Chairs. You can start this quest in the Sanctuary location after the completion of the side mission named Hidden Journals.
2) ECHO Recorder 1
3) ECHO Recorder 2
4) ECHO Recorder 3
5) ECHO Recorder 4
6) ECHO Recorder 5
Accept the mission from Tannis . You will have to gather all the ECHO Recorders marked on the map. The first one is behind Claptrap, on a shelf . The second one can be found in a dumpster shown in the screenshot . The third one is to the right of the entrance to Moxxi's . The fourth ECHO Recorder can be found inside Zed's clinic .
The last recording is beside the entrance to Scooter's workshop . Having collected all five, return to Tannis for your prize.