The recent question by tsaeb on this subject prompts me to ask the following:
Suppose a vehicle is rushing toward me at 100 mph, with a lot of sound emanating from its engine. I am standing still. Let's also say that the speed of sound with respect to still air as the medium is 700 mph. Now Relativity tells me that the sound must be moving toward me at 800 mph with respect to me , and the still air surrounding me, and yet, I hear no sonic boom, only the high pitch due to the doppler effect. Please explain.

well, first off the sonic boom doesn't come from sound travelling faster than the speed of sound (which it can't).
basically what it comes down to is that the speed of sound isn't dependent on the movement of the body making the sound. the air has a certain 'compressability' that only allows sound to travel at a certain speed. it just can't be compressed any faster, so sound won't travel faster.