Lazerdog spake, "Perhaps the only way to find MH370 is if the US Navy sends a sub into the area. They would have the technology to hear the pinger a long ways off. "
No, they wouldn't. Manned submarine and air launch sonobuoys are designed to listen for much lower frequencies from submarine and ship machinery that travel much further in water.
The FDR/CVR pinger operates at 37.5kHz with a maximum detectable range of about 3 nautical miles (18200'). Given a water depth of 12000' a surface listening device would have to be within about 13400' (2.2 nautical miles) laterally to have a chance of hearing it. Put a manned sub at, say, 1000 feet under an you increase that lateral range to 14500' (2.4 nautical miles). Even then you'd only "hear" it if ocean conditions were favourable. The oceans loves to bend and distort sound waves.
A deeply submerged ROV (say at 10000 feet) with appropriate equipment is the best option only when they have a good idea of the location.
BTW: In a past life I was the engineer overseeing software development for the AP-3C acoustic processing systems.