Most folks don't seem to mind the noise of their own bike, in fact many make them louder in the quest for power lol ... but I ain't one of 'em
The above suggestions are good 'uns, here's a lil more on the subject;
...and quieter doesn't necessarily mean a loss of power;
Bear in mind that no any one mod will silence the motor completely, much like performance modifications it teds to be a buncha lil things that add up. I'm a big fan of coating the inside of clutch covers with rubberized undercoating (automotive product, 3M brand is pretty good) for example - add Joe's mouse pad idea to quiet even more...