I was thinking more along the lines of screaming being a great way to use up all your oxygen. That too is an assumption though...

Not soundproof, but certainly sound-dampening. Quality speakers have an airhole to allow movement of air in and out of the box, for example.

Though this is straying a bit from the topic at hand, I must point out that quality speakers employ a range of designs, from simple sealed designs, to the ported designs you mention, on up to folded or full-scale (built with cinder block walls[0]) subwoofer horns.

The purpose of the port in ported designs is not just to allow movement of air, but to control the resonant frequency and Q-factor of the speaker. The individual dimensions, size, shape, and materials of quality speaker enclosures are all tuned to control resonance.

To sum up, there's a lot more to it than "an airhole to allow movement of air."

[0] I can't find the actual system I recall, but here is an older system with a similar design: http://www.burwenaudio.com/images/20000_WATT_HOME_HI-FI_0476...