The muffler consists of 3 pieces, the housing, baffle and outside cover. Mine was so bad that the cover was 1/2 missing nad the baffles was broke in half and had a piece missing. I just replaced the baffle and cover 2 nights ago. The housing was fine. Wow, what a difference. The mower is so much quieter and smoother. Like having a new one again. Mine is a 10201 silver series that I bought brand new in 1993 and I still love the thing.
While your in there make sure there is no build up in the exhuast ports which you can see after you remove your baffle. Just be careful becaus that is your piston on the other side of the exhaust ports so you don't want to scratch it.
The mufflers baffles has some slots on one side that the exhaust flows through. The cover has slots ont the other side of the mower that the exhaust flows through. That is what muffles the sound. I would guess if your cover looks fine your baffle is shot.