Am I right in thinking high frequency noise is blocked by high density material and not the low density foam type stuff. My friend was a sales rep for an sound attenuation company and I mentioned sound proofing he pulled out a sample of material about 10mm thick made out of bonded granite chips. It Was like a thick heavy tile that had a similar porous surface to a breeze block. So how crazy would it be to pebble dash the inside of a PC case. If I paint glue and sprinkle grit until 10mm acoustic treatment is acheived it would be a similar to the tile. These tiles cost like £50 per sq metre and I am sure I can buy granite chips from a pet shop like what you see in fish tanks.
I think this fits under the catagory of case modding.