Apparently this is common as dolphins have to make a concious effort to breathe so if their lives aren’t worth living they stop breathing – this has happened with dolphins in captivity when badly treated…

As for the Navy sonar kit, I am sceptical as to whether this would be the cause. The whole idea of this equipment would be to covertly monitor the presence of objects in the sea. If it caused a heap of dead marine mammals to turn up the shore, this is a bit of a giveaway..