I used to work for a guitar/bass/PA amplifier manufacturer as a designer. The test done on speakers for phase and functionality was pretty much the battery test described above. A "Thump" signified a working speaker and while watching the speaker you could verify the phase by the direction the speaker went when the test was run. This was especially useful for large PA speakers and subwoofers. The same effect often happens when the remote turn on of a car's amp is done wrong and there's a "Thump" through all the speakers when the amp is turned on when the car is.

The person speaking of a buzz is probably dealing with speakers with a bad speaker surround. Often the paper that actually joins the cone to the metal starts to wear and this is probably what is buzzing. I've had it happen with my own cars with rotted out speaker surrounds. They changed the design now on a lot of speakers to get rid of that issue i've seen, but they still make them with the paper surround. But as someone stated, DC with what is essentially a transducer sends the voice coil one direction only and to generate sound, it must be an AC type input. Currently I am working on designs in ultrasonics which are essentially voice coils that send the vibrations through metal at above hearing frequencies as opposed to a speaker which transmits audible sound through a cone to air.