Machinbird: sonar monitoring networks
I understand that fixed sonar networks were concentrated through the Greenland-Iceland-UK gap in the North Atlantic. It's a long way from there to the ITCZ. Moreover, the networks were operated to pick out and track long-duration machinery noise. The detection of the single, uncertain sound of an impact on the surface of the sea, in a noisy stormy region, even before considering the complex propagation of the sound far through the ocean, looks like a tough problem.
I hope that such avenues have been tried, but I am very pessimistic.
I guess the lack of volunteered information also means that no ship-borne sonars or radars detected AF447 either.
Maybe radar or radio signals from the descending AF447 could be lurking in someone's signals intelligence data, or a space-borne radar might have caught its descent, but I'm not sure that any cognizant agencies would be looking at the tropical Atlantic.