thrunthru :
Do you know anything about it? Or would it be good for a shot?
No, I don't. Though can say: a friend of mine tried to cover from inside his ordinary pc case with the material, looked like this one (don't know about its soundproof abilities). As the result, the temperature of pc components boosted up, and now he keeps case opened... like me))
Whatever, if you have nothing to do, or it's your hobby, you can try that Soundproofing Kit, but be ready for temperature rise rather than noise reduce.
I think your cm case is too cool to claim it so far. If you want silence, try to:
-listen to your system with drive turned off (if it is HDD). My system goes much more silent, when it happens (due to only ssd activity). If the result is noticeable, forget hdd the same as floppies, use ssd only;
-disable case fans (keep monitoring components temperature then);
-change HS for something with the only fan (140mm) with rps up to 1K (use real good thermal paste);
-choose graphics with a passive HS;
-...this fantasy list could be even longer, but that's enough for now))
The point is you cannot get a good soundproof and ventilation simultaneously, need to find compromise like: case with good ventilation + soundproof kit + low-heat components + minimum of low rps HS.
PS: if I'm not right, correct me using some facts, like 'I mounted water hs and..." etc.