Also I thought you need the negative part of the signal to produce the rarefactions.
Additionally, I see the term "AC current" thrown in a lot in discussions of sound. In this case does AC describe the voltage being negative at points?
The room/space cannot support constant pressure ("DC"), and if the speaker is not sealed, it cannot support constant pressure. In effect you have a high-pass filter (actually a band-pass filter) so the acoustic wave contains both rarefaction and compression, although the electrical signal does not have a negative component.
If you pick-up the sound with a microphone, the waveform will swing positive & negative.
You can get the same effect by putting a capacitor in series with the speaker... It creates a high-pass filter, and the speaker will move both forward & backward (from it's neutral position).
If you hook-up a battery (true DC) to a speaker, you'll hear a click when you connect it, and another click when you disconnect it. That's the filtering effects of the speaker and the air.
Even if there were a DC/constant pressure component, you wouldn't hear it because we can't hear down to zero-hertz, and the pressure wouldn't be enough to feel.