I have been wanting to do the same thing with my splitter that has a 9 HP Honda GX270 engine. I was not sure how much the muffler would make things quieter. I was not sure how much noise was exhaust and how much was hyrdaulic and/or mechanical. The splitters are certainly much too loud as they come from the factory.
I bought a muffler from a 250 cc ATV that should be much better than the stock muffler - these muffler come pretty cheap on eBay as the owners are putting on louder ones to get more speed (or just to make more noise and sound cool). I just need to fabricate a pipe to bolt the muffer to the engine.