This 'they heard the implosion at the same time/shortly after the sub lost comms' stuff..
And subsequent leaps to conclude that the US authorities 'knew' the sub had imploded days before anyone else...
Not what happened, at all.
Sounds were recorded, sounds in our oceans are being monitored in some places near constantly.
Those recordings do not have someone listening/watching them the whole time, and it is data that needs to be interpreted by someone or several someones who know what they're doing, and importantly, what they are looking for.
The ocean is not a silent place, and the Titanic is not a silent wreck, so noises from there are not unusual.
They were also not alerted that anything was amiss, that there was anything to look for until 8+ hours later.
They may have had a fair clue, far sooner than the public were told of it, that an implosion had occurred, but ' a fair clue' is not sufficient data to stop looking for a sub with five people on it.
It is also very normal that the public are not informed of every scrap of information about a situation - as this tragic incident demonstrates, most of the public haven't a fucking clue and will twist and misinterpret that information.
So no, no I don't think they knew for certain the sub had imploded straight away, I don't think that was ever possible.
I do think by the time the first remote sub with the capability to get right down to the bottom was deployed, they had a fairly good idea what they were looking for though.