I am not familiar with the muffler you have, but some mower mufflers have a baffle between the inlet and outlet. The baffle is just a piece of sheet-metal mid-way between inlet and outlet sections, at right angles to the gas flow. The exhaust pulse enters the muffler from the inlet, strikes the baffle, bounces around a bit, finds its way to the opposite end of the muffler on the inlet side of the baffle, and goes through a hole there to enter the outlet side of the muffler. The baffle is responsible for most of the muffling effect. After a few years (or sometimes a lot less than that) the blasts of hot exhaust gas may eat their way through the baffle, leaving the muffler as just an expansion box, so the mower starts to sound like the average Harley-Davidson motorcycle. (It is rumoured that there are only six sets of Harley mufflers in the whole of Melbourne: a pair is briefly lent to whoever has just received a canary and must front up for an exhaust noise test.)
You can easily check whether you have a perforated baffle by removing the muffler and looking in through its inlet port (that is, the hole where it attaches to the mower engine exhaust port). If what you see through the port is a piece of tin with a fairly ragged hole through it, you will know what is wrong, and what to do about it.
