Another interesting possibility is that rather than pointing your sound at where the fire is now, point it at where it is about to go. What may not be able to put out an active fire may be able to suppress its spread, which would also be an enormous win in many circumstances. Even if it could just significantly retard the fire while conventional techniques are deployed in combination, it could be a potent addition to the arsenal.

To be honest, I'm sort of skeptical, especially of the "glorified speaker" approach, but as it happens there are ways to produce some very loud noises at 30-60 Hz which is very well-established off-the-shelf tech: http://epb.apogee.net/res/rehpuls.asp This used to be a standard furnace technology in American homes. Much like cars, it would be tuned to be as quiet as possible (which I've heard could still be annoyingly loud), but just as I'm sure you've all heard a car without its muffler and marveled at the difference, I'm sure something based around this technology, only meant to be as loud as possible, could achieve some serious decibalage, quite efficiently. Probably a great deal more efficiently than converting something to electricity and then converting that to sound.