How does the center tap terminal work in Knowles Speakers ED series?
"a positive going voltage relative to this center tap can lower the output volume". 

The datasheet doesn't actually say that. It says that "a positive going voltage... will cause a decrease in pressure at the sound outlet". I think that all it is saying is that when you apply a positive voltage as specified the internal diaphragm / speaker effectively moves inward causing a reduction in pressure at the sound outlet. You need to drive this unit with an audio signal exactly as you would a speaker.