In a PC, there are several sources of noise. Reducing speed and decoupling can assist in reducing these, but adding housing insulation can help with removing noise entirely.
Mod King are high quality mats produced in Germany, and are made from soft polyurethane acoustic foam, with a top cover of finely perforated leather material, which is resistant to dirt and can be wiped clean. The underside is self-adhesive.
Sound absorption is most effective at 1800 to 3500Hz. Between 1200 and 6000Hz are reduced also to an extent, and the high frequency sound of many fans will be reduced significantly.
Installation should be started first with the biggest parts - it's a good learning point for the installation and also that the smaller parts later may need to be cut into smaller pieces (use a carpet knife or sharp scissors). Leave a 1 to 2cm margin to allow enough space for the panels to close.
Closing unused slots or fan vents can help lower noise output but take care not to restrict flow in a hot system. On tighter / winding areas, it's often easier to cut the material into rectangular sections rather than apply one large section. For cases without a front door, it's recommended that unused external drive bays have material installed on the inside.