That muffled sound is the lady of the house bound and gagged in the bathroom.
Hearing a cell phone ringing, a man walks out of a room to answer it. The caller asks for Caroline, the lady of the house. He tells the person that she cannot come to the phone at the moment. As he asks the caller to leave a message, Caroline appears and it becomes obvious why she cannot talk on the phone. She is sitting in the bathroom and she is tied up with rope. Her mouth is stuffed with a panties and a rag tied over her packed mouth to gag her. Although the gag keeps her muffled and the rope has her secure, the man is not satisfied she is completely helpless. So using more rope, he ties her tighter. He removes the over the mouth gag, re stuffs her mouth with a sock and ties the same rag tightly between her teeth to gag her better.He then removes her skirt, puts Caroline on the floor and leaves her to struggle bound and gagged in her girdle. After a while, the man is ready to make his get away. He returns to his captives, pulls her top down and hogties her.