I just beat P2's single-player mode a few days ago (I'm still waiting on an opportunity to try multiplayer though, seeing as I have dial-up and as a result my Steam account spends the majority of its time in offline mode). So...who else is excited for the Perpetual Testing Initiative?
Oh, and you can find the quartet of singing turrets in Chapter 3, Test Chamber 16. You'll have to disable the turret in the vent first, though...