Cecil first notices the roar when it becomes loud enough to drown him out. He's in animated conversation with someone-- caught in the middle of some expansive gesture-- when he realizes the extent to which having to shout over the train-passing-overhead level of noise is taking the nuance out of his point.

He frowns; opens his mouth to shout ("Was this planned?"); and is left blinking in the sudden plunge into blackness. His original point is replaced with a surprised "...oh."

...Well. Someone's got to take control of the situation. His ears are ringing in the startled silence, which he knows must come seconds before the inevitable chaos. Time is short.

"Listeners!" he begins, and quickly amends that to, "...That is-- fellow guests. I implore you: Do not panic! Remain calm."

He has no idea who's listening, if anyone; he can't even tell what's happened to the circle of people he was standing in. They're just their voices now, and their voices are silent; until the lights come back on, he thinks, they will all exist only in the act of being heard. (...Cecil hopes more fervently, following on the heels of that thought, that someone was listening.)