Much of what Margin Call is about has already happened in the few days and weeks before the day in the movie — the events that precipitated the widespread firings, including a rather deliberate firing of the head of risk management, but are not really discussed.
In the meeting, Sullivan is talking about the music slowing, saying "if the music were to stop…". He's suggesting that the situation is bad, in the context of the un-discussed preceding days, so some action is needed, which is why he escalated.
Tuld saying he fears he "can't hear anything" is about much more fundamental uncertainty — his own belief that no set of numbers could be useful (he didn't even want to discuss the numbers in the report) and that the actual situation required acting on his gut instinct that it was much, much worse.
He, basically, thinks the risk cannot be assessed ("I'm telling you, this is it!") and arbitrary action is needed.
In short, the point is he did not have any confidence they could ride out the consequences of the preceding, largely un-discussed few days, because they were beyond their parameters, and that there was no time to assess the real hidden risk.
The trigger for the chosen action is not the newly correct numbers in Sullivan's report, but it forcing Tuld to confront his belief that even those numbers were no guide for anything.
(Sam's dog dying after his sad failure of a divorce is a crucial metaphor — the dog days are literally and metaphorically over)