On this page of the game guide to Borderlands 2 you will find the walkthrough description of the main mission titled The Man Who Would Be Jack. You can begin this quest in Sanctuary and it will require you to travel to a location named Opportunity.
2) Jack's double/Info Kiosk
3) Info Kiosk 2
4) Info Kiosk 3
5) Info Kiosk 4
6) Info Kiosk 5
Speak to Roland  and head to The Highlands Hyperion Bridge. Afterwards go to the bridge in the western part of the map where you should find a passage to Opportunity. There, look for Jack's Double  and kill him. He will have some guards underlings with him who you can use for Second Wind. After defeating him, pick up his Pocketwatch.
Now approach the Info Kiosk nearby where you fought with the Double . Listen to the recording and head out to find four Kiosks , , , .
After you collect the voice samples, you will have to send them to Angel. In order to reach the computer, you will have to go down to the underground passage. The tunnel entrance can be found by the spot marked as . As you're transferring the files, the room will be attacked by enemies from time to time. With the process complete, take the Voice Modulator. Now you can return to Roland  in Sanctuary to finish the mission.