It's interesting to see that many whales produce sound bursts in the region of 20kHz - but some species produce sound bursts up as high as 40kHz.
Seeing as the signals picked up by the Australian Navy were reported as being around 33kHz, I wonder if this might have been sound bursts from a whale?
Whale sound bursts are reported as travelling up to several hundred kms. The Cape Leeuwin hydrophones of the CTBTO are constantly being bombarded with whale signals.
Whale signals detected by International Monitoring System (IMS) facility: CTBTO Preparatory Commission
One has to ponder whether the major sounds produced by an aircraft ditching into the open ocean would be airborne or waterborne sound, as each has to be picked up by different methods and different instruments. It's interesting to note the CTBTO hydrophones pick up waves crashing, as permanent background noise.
The CTBTO has already stated the Cocos Island infrasound station did not, and could not, pick up any airborne sound from the region where MH370 is supposed to have been lost, due to extreme distance.
Acoustical Society of America - 166th Lay Language Papers