I think there is still hope, but the one thing that would completely explain everything except the banging noises - loss of communication and surfacing capability would be a catastrophic failure days ago.
If they are still alive, may God have mercy on them. After four days down in the cold with probably little food to keep the metabolism up, they would probably start shivering, and this involuntary muscle action will deplete oxygen which they try to preserve by controlling breathing.
I am not sure if this was posted yet; if so apologies, but a prior passenger claimed the batteries died after just one hour and communication was lost for hours...
A YouTuber who once traveled to the Titanic on the missing Titan sub shared how the vehicle's batteries suddenly drained during the expedition, forcing it to end early.
www.dailymail.co.uk
I'm reminded of very long ago, during my elementary school days (which is kind of tricky since school had not been invented yet when I was that age, and for that matter, neither were books or other reading material) of learning about a kind of deep-sea craft called a bathyscaphe. A detail that I remember from way back then, and just confirmed now by looking it up, is that the bathyscaphe carries a load of expendable ballast held in place by electromagnets. To ascend, it just cuts the power to the electromagnets, dropping the ballast. A fail-safe aspect of this, of course, is that it takes power to hold the ballast, and if power is lost for any reason, the electromagnets fail, the ballast is dropped, and the craft surfaces.
I have, also a radio-controlled toy submarine, which has a sort of a fail-safe way to surface as well, but not one that'd be practical for a real, manned version. To submerge, it depends on a downward-facing ducted propeller, to pull it under the water. It has ballast that can be added or removed, to tune it to just the right amount of positive buoyancy, so that when the submerging propeller is not running, it naturally floats to the surface.
A very neat toy but an economic failure. I guess it just cost too much to manufacture, to be able to sell for any price that anyone was willing to pay. When I first saw it in a hobby store, it was priced at something over $300. They dropped the price to $229.99, and it still didn't sell. Then they dropped it to $50 (surely a lot less than the store paid for it in the first place), at which point I snapped it up.
I guess I wonder if any similar principle was applied to the missing submersible, of having it be made in such a way that it required something actively functioning in order to remain submerged, such that in the event of any kind of failure, it should be positively-buoyant and float to the surface. The bathyscaphe was invented in the 1940s, and so even that far back, the idea of a failsafe system based on requiring something to be actively functioning in order to stay submerged was well understood.