If a sound is muffled, you can tell something is making noise, but you cannot hear it clearly — as if a door, a wall, a coat, or a blanket is sitting on it. A voice in the next room, a phone in a bag, drums under a cloth: they come out dull and round, not sharp. This is ordinary English for sound, not slang. People also say someone is muffled up when they are wrapped in scarves, but the sense here is about hearing. Though actually, writers use it for feelings too — a muffled laugh, muffled anger — meaning the thing is held in and not fully let out. The opposite feel is a sound that rings out clear.