I'll throw this out there too, many compressors are mounted in the corner of a shop, which acts like a megaphone.
Go get yourself some acoustic ceiling panels from the homecenter, like they use for ceilings in basements, and nail, glue, or staple them to the walls surrounding the backside of the air compressor...this will absorb the noise being put off by the belt/pump rather than re-projecting it back out into the room.
Seen a guy once run a line from his intake up into his shop attic and over to where the filter would be easily cleaned/replaced by the attic access door. Compared to what a guy is used to hearing with these large vertical's it was noticably quieter, would guess about half as loud as i was used to hearing...i do like the muffler deal.