The voices call Bob to come see something, and he's very nervous, and it takes a good long time, and it's fairly hair-raising, and they keep going, "Come and see this! Come and see!" You knew something fucked up was going to happen to him the second the Doctor took time out to patronize him. No, scratch that: The second you saw how pretty he was.
So maybe the Aplans died out 400 years ago because the Angels got to them, at which point they moved into the Catacombs and took the form of their statues or something. They look like regular statues and not Classic Weepings, and are moving dreadfully slowly and not super-fast like usual, because "They're dying. Losing their form. They've been here for centuries, starving. Losing their image. And their image is their power. Power. Power!" Why are you screaming? Oh, because he just figured out that they're in the middle of a waking, hungry army of orthodox images, dead religion. That's scary. Mushfaced near-dead ideas that don't need us anymore. The Bish calls for Bob and Angelo and Christian and whatever, because the rest of the twenty are with the team, and Bob's voice evangelistas over the walkie: "I know, sir. Angelo and Christian are dead, sir. The statues killed them, sir." And he is heading back up to us.
"Well done, Bob. Scared keeps you fast, told you. Your friends, Bob, what did the Angel do to them?" Snapped their necks. Everybody discusses that that is weird, because of the time-displacement thing they usually do, but apparently they probably "needed the bodies for something." Octavian and the Doctor fuss for a second before Bob explains that he did not escape them. "The Angel killed me, too. Snapped my neck, sir. Wasn't as painless as I expected, but it was pretty quick, so that was something."
And then -- as in I'd say half of the episodes of this show -- we realize the voice of the innocent person we barely met is actually the voice of the monster. "You're not talking to me, sir. The Angel has no voice. It stripped my cerebral cortex and reanimated a version of my consciousness to communicate with you. Sorry about the confusion." Which means the Bob that is approaching them -- the dataghost, as we'd say of the Vashta Nerada -- is the Angels getting closer. Which they already were. Everybody screams and runs about in the caves, flashlights joggling all over the place, and the Doctor apologizes to Octavian for bitching at him but, he points out, the guys were already dead anyway. "I know that, sir. And when you've flown away in your little blue box, I'll explain that to their families."