A muffled sound is quiet or muted. You might hear the muffled sound of your neighbors laughing through the wall of your apartment.
If you don't want your brother to hear you cry after you finish a sad book, you might put your covers over your head, so all he'll hear is muffled sobs. And if your math class is giggling at your teacher's strange outfit, your hands over your mouths will produce muffled laughter. Muffled comes from muffle, "to cover," which probably stems from the Middle French mofler, "to stuff," and the Old French moufle, "muff or glove."