Builders, enthusiasts, and modders, lend me your ears....I come not to bury sound but to deaden it. All that sound deadening material (no matter what you use, is also an insulate. That means it traps heat. Most cases are made of metal and this gives the case the advantage of being a huge heatsink, so to speak. Sealing this heat transference with sound insulation, places a lot more focus on the small fans which exhaust it.
There are many things a builder can do to minimize the decibels being generated in the first place. Using rubber feet to anchor the motherboard is an excellent way to reduce the vibrations which are transferred to the metal case. Using cooling fans that produce low noise are good, but remember that many fan specifications state decibel levels not being attached to the heat sink or anything else. When they are attched the sound level increases, so have a care when studying the specs on those guys. I have heard computers that sound like a freight train, and after examining them, some components are loose, the screws are not tightened sufficiently, or they just do not fit well together. These things can, of course be rectified, merely by making sure things dovetail together properly. Power supplies are a huge source of sound so study the sound qualities of your PS before you buy and use rubber or silicon strips to insulate it's vibration from your metal case which acts like an amplifier. Lastly, many case fans can be significantly deadened by using rubber inserts instead of metal screws. I am certain there are many other techniques which result in lower decibel levels.