I would assume this sort of technology requires line of site (or whatever it is called underwater). I am guessing that the previous incident you are talking of happened in the Atlantic? Where it is thought this aircraft went down is almost surrounded by land, I do not know how the sound wave would get around these masses.
Sound propagation in the oceans is a complex process. If you want the details along with some math to back that up, there was a thesis link a few hundred posts back. The short of it is that the ocean's surface and bottom both reflect sound waves in a complex manner, resulting in channeling of the acoustic energy in what amount to a wave guide. Further complications include wave bending due to differences in water temperture and salinity. Signal loss in the channels is inversely proportional to distance, not the square of the distance as in line of sight, so signals can and do travel very long distances.
The hydrophone networks (e.g. the USN's SOSUS) were initially placed at choke points like the GIUK Gap to detect transit of Soviet submarines but proved able to track submarines as far away as the US continental shelf and low-flying Soviet patrol aircraft, all back in the 1960s. The capabilities of current systems are not publically available but I'd assume that the massive improvements in sensors, underwater cables, and computing power have produced near-realtime knowledge that might be applicable to the current search. The problem is that providing such data provides a lower bound on what the system is capable of.
The issue of propagation in shallow water interacting with surrounding land masses is complex but of critical interest to both civilian and military sectors due to the hunt for natural resources and the focus on littorial warfare. Lack of public disclosure for the supporting evidence behind the current search areas is hardly surprising.