There are many ways to quiet down a mower, though many of the options will cost you some money.
A cheap way is to seal up the gaps between the exhaust pipes and the muffler itself. This should quiet it down quite a bit.
Another cheap solution is to purchase exhaust gaskets that match the engine. Your exaust gaskets may be worn out, so replacing them with new ones should improve the sound quality.
The more expensive solutions is the buy a whole new muffler. You muffler may have holes on the outside or in the inside.
