They need a FAQ for this...
Sound deadeners such as dynamat, b-quiet, asphalt, vinyl. Really do nothing for you within a computer case. There main purpose it so add wieght around a certain area in effect reducing vibration. I see ppl lining up there doors with it which a waste of money, its not designed for soaking up airborn noise, its design to reduce the noise before its created..vibration. If you were to use it you would only need to use in a few spots around HD, or fans, but you can practicly achive the same thing by using rubber gromets or rubber washers, without the mess and unnessary wieght. For a test put your hand over a peice of sheet metal where you think vibration is, what you get by putting a little wiehgt on that peice of metal is basicly what a sound deadener does for you.
For noise from a spining harddrive, loud fans and such, you would need something that would soak up the noise on impact, such as an acoustical foam. Somewhat opposite of a sound deadener because its light wieght and doesn't reduce vibration, most sound deadeners reflect sound, ex: concreate parking lot (dynamat concreate/lead based). In something like a car door and floor panels, where you would want both worlds you would slap on the sound deadener first, multiple layers if wanted, then you put acoutical foam on top of that.
Its funny to see ppl line up there case in sound deadeners, and be all pissed it does nothing, no sh|t retard..do ppl read instructions anymore? what a mess what a waste.