check out car accessories for sound proofing materials ..

normally it's neoprene ..
if you're ghetton modder, those 30 eggs cardboard can muffle sound as well .. lol

just walk in those car accessories .. ask them politely for sound proofing materials .. they'll show some example .. pricing

those materials wont absorb heat .. coz they're petroleum byproducts (something like our plastics)

if it's bout sound proofing .. since most sounds like caused by fans ...
by changing a larger fan (120mm~140mm) and run it a lower voltage is more reliable ... not just it runs cooler, it run quieter

80mm (8cm)
3.142*4*4 = 50.272 cm²

120mm (12cm)
3.142*6*6 = 113.112 cm²

1x120mm fan > 2x80mm fan

hope this helps