Mufffler re-pack 101
Over time the packing material breaks down inside the muffler and normally causes excessive noise and you can lose a bit of bottom end power..
Most decent bike shops sell the packing material for about $35.00 pack [note- get the sheet type(road bike) not the cotton wool looking(dirt bike) packing.
you can also get the stainless steel wool sort that Birdman mentioned - lasts longer is all.
Then all you need is to drill out the end cap rivets , extract the end cap /core and remove what is left of the old packing.
When you have trimmed the replacement packing to size (tightly wound = loud, looser = quieter) re assemble and refit end cap using ULTRA GREY RTV sealant between the end cap/body and secure with new pop rivets... Presto..
Hint: wheel off makes it easier... piss easy if you have a centre stand.