The lack of reports of Engine noise by any of the Bar Patrons suggests the Engines were not running. There are several reasons why that might be.
SASless, this isn't a very useful attempt at analysis.
Bar had patrons, talk, noise, a band, and the acoustic buffer of walls and a roof to mute any sound form the outside -- be it a car, helicopter, or an elephant's mating call.
Bar patrons' attention was most likely on what they were doing, or the lovely pair on that gal over there ...
... I'll venture a guess that they were not silent, ears atwitch, listening for a helicopter engine sound/noise.
With the speed at which this event looks to have taken place, any noise form above would first be an interruption to what they were doing, followed by a "what's that noise" sort of response.
Their alert cue to the presence of an unexpected helicopter was the thump on the roof. Attention getter, no question.
What the bar patrons didn't hear in terms of engine noise is irrelevant.
Their attention was not on what was outside and above them until after the thump.
By the way, I am pretty sure you are aware that the first cue most of us on the ground have of a helicopter flying nearby or overhead is rotor noise, not engine noise.
The T/R malfunction videos posted, the one in NY and the one that disappears behind the trees, shed some light on what can be walked away from, and give some weight to TC's point on the last phase of this descent being high RoD ... since they didn't walk away from it.