A french submarine is sent to investigate and finds nothing. Said submarine can find another sub in the worlds oceans, but can't find a data recorder (that wants to be found, due to emitting a signal on a specific bandwidth) that is more or less right next to it
So it's that easy then?
If you have any kind of search abilities using Google, you will be very capable of finding a report where two of the latest, highly instrumented, sonar overloaded submarines (one British, one French) HIT EACH OTHER in the middle of the Ocean.
Both submarines were equipped with state-of-the-art sonar technology, but xxxxx said it was possible that neither was aware of the close proximity of the other vessel.
So, your implied assertion that it is simple to find an audio signal (which incidentally we do NOT know was functional) using a sub is perhaps a little off. If two submarines can get close enough to HIT EACH OTHER, with all this fine sonar technology, which is specifically intended to detect other submarines, and not even know the other was there rather clearly confirms that acoustic detection at best is a hit or miss affair.
The search area was, what, 3,200 square miles... with depths that exceed the hull crushing depth of most submarines.
Modern nuclear attack submarines like the American Seawolf class are estimated to have a test depth of 490 m (1,600 ft),[which would imply (see above) a collapse depth of 730 m (2,400 ft).
[Wiki]
That leaves a LOT of water and many thermoclines between any sub and the bottom of the Ocean in this area.
'Nuff said?