Curtin University Acoustic Signals
Ornis, you may be correct, its hard to find accurate sea surface temperature measurements for the 8th of March and it also depends on whether the plane hitting the surface caused the signal or the plane imploding under the ocean caused the signal. The acoustic sensor is located at a depth of 400m so this affects the speed as well.
Using the image Minimbar posted the final ping is about 1600km away from Perth, so this would make it 17mins for the travel time of the signal and a further five minutes the plane would need to glide (54mins).
ETH961 glided for 27mins after last engine shut down from 20,000ft, (even with hijacker interference).