To make a sound quieter and less clear. (Cambridge Dictionary)
To make something less strong or clear. (Cambridge Dictionary)
To wrap up so as to conceal or protect: Envelop. (Merriam – Webster)
Wrap or cover for warmth. (Oxford Dictionaries)
Cover or wrap up (a source of sound) to reduce its loudness. (Oxford Dictionaries)
Make (a sound) quieter or less distinct. (Oxford Dictionaries)
If something muffles a sound, it makes it quieter and more difficult to hear. (Collins Dictionary)
Muffle definition by the Free Dictionary.