the easiest way to quiet a muffler is to take it off!
hehehe
ok really, go buy a couple of Stainless steel kitchen scrubbers, and mash them into the muffler, on my supertrap I put them in from the exit, but I had a removable baffle to keep them in, if you don't have a removeable baffle, put it in from the inlet. they do this with bikes, alot of people use steel wool (but it rusts and blows out, but at sams club you can buy 12 SS scrubbers for 8 bucks, and it won't rust and its larger wire won't blow out as easy.
be careful about packing this stuff in too tight, as more doesn't always mean better. this is the same premise as a glasspack,
this would also then make your muffler "spark arresting" which is always a good thing