Sonar has picked up banging noises, maybe they might just get lucky.
Alas, I think that pinning any hopes on sounds that may or may not have come from the sub is pointless.
If it had been a military boat that had sunk, then it might be easier to isolate the banging using passive Sonar.
You just wouldn't believe some of the sounds that eminate from subsurface, it really is mind blowing....!
Much that I would like to attribute 'knocking' to the stricken craft, and therein believe that the passengers are still alive, unfortunately I can't see it.
If a military boat is reported as 'subsunk', then there would be a distress beacon released (assuming there hadn't been a catastrophic event rendering that impossible).
At least, SAR units would have a good indication of where the boat was.
For some reason, the operators of the submersible don't appear to have any sort of distress beacon available, but that's for any subsequent inquiry to establish.
At the depth that the submersible is reported at being, then sounds can come from anywhere (literally), because sound transmits well through water. Unfortunately things like salinity, water temperature, thermal layering, sea state, and such can bend, and distort the noise, so locating where it originated becomes incredibly difficult, even with the most sensitive of hydrophones.