he main problem was that the recorders pingers could have malfunctioned.
BEA says the detection range of the Underwater Locator Beacon (ULB) also called as Emergency Locator Pingers (ELP) is up to 2,000 m.
The manufacturer's technical brochure does give a detection range, but says the emission covers 80% of a sphere.
That would mean the microphone would have to be lowered by over 2,000 m to have had a chance at locating the pinger.
http://www.benthos.com/pdf/elp362D_001815_revK.pdf