Synonyms for Muffled (same or very similar meaning)
WordNet sense 1 (overlaid or spread or topped with or enclosed within something; sometimes used as a combining form):
coveredWordNet sense 2 (being or made softer or less loud or clear):
dull, muted, softenedWordNet sense 3 ((of sound) relatively low in volume):
softFrom the ODE community, based on WordNetadd/edit