What does the capacitor do for the speaker? I don't have to worry about the cross over frequency since I don't have a 70v system right?
The capacitor functions as a first order high pass filter.
Since you have a limited frequency response driver in that horn that has no built in protection against low frequencies a crossover (or some way to limit the amount of low frequency) reducing the low frequency to the driver is required or you will damage the driver itself. This has nothing to do with the speaker being run through a 70v transformer or not. It is about the speaker design itself.
The rating of the capacitor would change depending on the tap that you use on the speaker input. This is shown on the table at the bottom of the document that I linked. This is also shown there but to emphasize it "Use non polarized electrolytic capacitors rated at 150VDC or greater, wired in series with the positive lead of the speaker line, one per driver."
Lee
