Where can you hear the vault 101 PA system in Fallout 3

You can hear it after you beat the quest "Project Purity." You will hear a distress signal from the vault and if you listen to it you will get a new quest, "trouble on the homefront" once you walk into the door of the vault you will once again hear the Vault 101 PA system, not that there's much to hear. p.s. you have to be somewhere near the vault such as in Megaton to hear the distress beacon.