The more I read about the USN SOSUS system and about the nature of the SOFAR channel for transmitting sound long distances without loss, the more convinced I am that positioning data on the AF447 accident can be recovered if someone can be convinced to make the effort.
To understand what the SOFAR channel is about, look at this link:
DOSITS: History of the SOFAR Channel and then follow the internal links for more information.
The SOSUS system has been simplified over the years to reduce manpower requirements and has apparently lost some functionality, but appears to be still in existence.
Remember that the AF447 accident is a multi depth event. Even if surface noise may not carry well, it may reflect from an undersea mountain that is in the vicinity and then be carried to a hydrophone in the SOSUS array. Maybe the sounds of the engines hitting the bottom will carry.
We have the advantage of knowing within relatively small windows the time of the crash and the location of the crash. This permits writing algorithms for summing up the sounds within a particular directional hydrophone array to achieve higher gain from a particular direction, and for predicting the arrival time of potential signals at individual hydrophone arrays thus permitting more detailed analysis. Once the accident sound signature is identified on one array, it can be more easily found on the signals recorded on another array in a distant location. Once you have signals on a second array nailed down, you can use that information to predict with loran type precision the coordinates of the events detected.
It is probably worth a shot. It would take some analysis to come up with results and it won't happen unless someone tasks the Navy for results. An inter-governmental request by the French government would be probably all that is required. I hope they are working on it.