To clarify my post, here's what you do to silence the intake noise;
Remove the stock airbox. If you can weld, fabricate a steel flange and pipe the same diameter of the lawn mower muffler. Or you can buy an aftermarket velocity stack made for scooters or large scale r/c models that use Walbro or Zama type carburetors. There are stacks for other oddball carbs too. They all are designed to take a Uni type air filter, but instead of putting on an air filter, using a short length of radiator hose, connect the velocity stack to the EXIT side of the muffler. That's right. Put it on backwards. For some reason, and this is the secret, it quietens down much better when the air is being sucked through the side that the smoke would come out on a lawn mower.
Now, on the other end of the muffler you will have the threaded end of the muffler sticking out. You can cut it off to make the assembly more compact or leave it and simply slip a Uni-type air filter over the OD of the muffler, covering up the threaded end.
Try it with the threaded portion on first. That's how I did mine and it made a Dramatic difference in the noise coming from the engine, so I never cut the threads off. I mean it made a DRAMATIC difference. Its hard to believe all that noise is coming from the airbox, until you think about how a 2 cycle engine works. Large 2 stroke engines like on a dirt bike have reed valves in the intake tract that shut and stop the sound pulse that comes back out of the air filter on the little lawn equipment engines.
Now, if your choke is integral with the original airbox, they make chokes that sandwich between the aftermarket velocity stacks and the carb. Its a sliver of aluminum with a thin steel blade that slides. They are for the scooter and r/c engines too.
Here's a link that has the velocity stacks, chokes, and filters all on one page!
http://www.davesmotors.com/Products/AF-Air-Filters
Here's one for the muffler.
http://www.tractorsupply.com/en/store/arnold-replacement-muffler-for-briggs--stratton-89966
You'll have to find the piece of radiator hose and hose clamps yourself!
Trust me. The noise emanating from your airbox is 10 times louder than that emanating from the muffler on your weed trimmer. I'm serious. This trick made my engine 10 times quieter!