As I was walking today, I was thinking about this illusion, and one of the arguments that people used to question the "legitimacy" of it: how could you hear Criss Angel when he was on the other side of the glass (before the illusion happened)? My answer: wireless microphone, like you see people wearing for interviews. The camera crew has a boom mike to record the people on the street (you can see it in the reflection of the window at one point). Does that answer how he did the illusion/trick/magic? No, but it answers (hopefully) that point.
It's a cool trick, I have to admit, and I tend to be one of those people who tries to figure out how these things are done (usually completely unsuccessfully though.....)
Al
Edit: I just re-read the posts, and realized what was mentioned (how buddy who came up to touch the paper was able to hear Angel perfectly) isn't how I initially interpreted it. Anyhoo, it's all about "the hand being quicker than the eye" and the illusionists always have you focussed/distracted by one thing or another, so that's probably part of the routine (throwing the viewing audience off with minor details). Still a good trick, especially on repeat viewing (I was looking for window to move/shift/etc and didn't see anything).