MountainSnake
"On 5 June 2009, the French nuclear submarine Émeraude was dispatched to the crash zone, arriving in the area on the 10th. Its mission was to assist in the search for the missing flight recorders or "black-boxes" which might be located at great depth.[93] The submarine would use its sonar to listen for the ultrasonic signal emitted by the black boxes' "pingers",[94] covering 13 sq mi (34 km2) a day. The Émeraude was to work with the mini-sub Nautile, which can descend to the ocean floor."
Source Wikipedia.
Are they trying to listen to the FDR/CVR pingers in the area where the transponder stopped emitting? Maybe they are but I have never heard anything about yet.
There are a lot of naval and air assets out there. Some have an anti-submarine warfare capability and with it, an underwater acoustic capability. The areas west and east of the Malaysian peninsula so far searched have consisted of shallow water. Any continuance beyond the Straits of Malacca starts to run off the continental shelf to deeper water in the Indian Ocean.
Shallow water makes for a notoriously difficult acoustic environment. Those waters are chock full of fishing vessels. Man made noise and natural noise in the water adds to the ambient noise, which will be much increased over that of an open ocean environment. Reverberations will be rife. There will be surface reflections and bottom bounce (depending what the bottom is made of).
In deep water, sounds spread spherically, greatly dissipating the intensity. Shallow water, constrained by the surface and sea bed, causes the sound to spread cylindrically, meaning in a given volume the noise is greater. It's logarithmic and I can't be 4rsed digging out my old notes! 10Log versus 20Log.
That said, the shipping noise is low frequency whereas sonic locators on Flight Data Recorders is not. Regardless of the frequency, ambient noise is still a factor and higher frequencies are subject to greater attenuation than low frequencies.
In the Air France 447 case, the location was open ocean; much quieter acoustically and a submarine could manoeuvre in the deep water there. Submariners are notorious liars about where they are and where they've been but if they tell you they are loath to go anywhere a prang might ensue, I'd (guardedly) believe them. They certainly don't like shallow water.
They could use a surface vessel with a decent SONAR suite to listen for the location device. To cover any area AND listen is almost mutually exclusive. They may need to sprint and drift, otherwise their Own Ships Noise (OSN) may mask what they're listening for. Otherwise, drop sonobuoys from fixed or rotary-winged aircraft. That might do it.
Look, an acoustic search really requires a half decent datum to start with. An acoustic search of the South China Sea or the Indian Ocean, or even a corridor of the Indian Ocean, is really not a practical primary search tool. Searching for this aircraft now, entering the sixth day with so many false starts, will have left any traces on the surface in a much diminished state.
I flew SAR on Air India 182 back in 1985. I was an acoustic specialist on my Nimrod Maritime Reconnaissance crew. We dropped buoys but heard nothing. The water there was over the 1000fathom line and was "quiet". Plus we dropped buoys at each of the 2 datums of wreckage on the surface, so we were as near to the source as one could be, under the circumstances. We heard nothing. To be fair, sonic location devices then were in their infancy and I've no idea if the Air India 182 aircraft had been fitted out with such a device.