I recently came across some form of a bug or glitch during this quest. When the Grey beards ask you to demonstrate your voice against the summons due to some strange reason the summons would not appear.
Even if you cast in the general direction they were still not there to harm. This is my 5th play through and this is the first time this has occurred so no quite sure of the cause.

I did recently how ever discover that pressing Start/Pause then returning to the screen would make the summon appear. This only work if you directly pause the game, pulling up no other menu had the same effect. You are then able to shout at the summon and move on.