I just watched a couple of docu's on Youtube about Scorpion and Thresher. They were very good. They talked a lot about the SOSUS and how they were able to locate Scorpion with it.In 1968, the U.S. nuclear sub "Scorpion" imploded after an explosion of some kind, and the energy pulse was "heard" by hydrophones in the Atlantic. These had to be amplified in order to triangulate the approximate location of the wreckage. Again, in the 1970's U.S. hydrophones in the Pacific "heard" the Soviet sub K-129 breaking up as it fell three miles to the ocean floor, and used this data to not only locate, but recover the sub. Rent "Azorian" for an amazing telling of that story, well illustrated, too!
Insofar as 1912 was concerned, there would be no device, and certainly not human ears, which would detect impacts and implosion at a depth of 4KM. If a conscious ear was under water in total silence, I don't doubt collapses and rending of steel would be heard as tiny crunches. Nobody in his right mind would have stuck his head into that sea for that purpose, plus, as is pointed out above, the noise on the surface would cover all undersea vibrations.