The screams were muffled by the dense foliage.

His footsteps were muffled by the soft grass beneath them.

A synonym for "muffled" is silenced. An antonym for "muffled" is unblocked. Please see the related link below.

He spoke in a muffled voice as he did not want to be overhead. The sound of the music was muffled by the closed door. The thief attempted to steal the jewels with muffled footsteps.

The children were muffled up against the wind and snow.

The sound of the orchestra was muffled by several closed doors between the concert hall and the corridor where I was standing.

IT means that a noise is a bit covered. e.g. "he said in a muffled voice."