A sonogram like the one in this app will confirm that the effect is physical and not just an illusion: https://play.google.com/store/apps/details?id=com.ntrack.tun...
What should I be listening for in the video? There's the moment at 0:45 when everyone stops and says "wow", but that could come from either explanation.
Should I be expecting to hear a pitch that's higher than all the notes in the chord, lower than all the notes in the chord, or a different phenomenon that's not a distinct pitch?
This effect is sometimes claimed to happen with a 4:5:6:7 dominant 7th chord. In that case, would I hear it at the 28th harmonic? The 30th? The 420th (which is the lowest overtone that all the notes have in common)? The 1st (the undertone they have in common)? Or is 4:5:6:7 actually a bad example?
(I really doubt it's the 420th, of course. That would be a very high pitch around 17 KHz, which would be difficult to hear, and wouldn't be preserved at YouTube quality anyway.)