His bookkeeper is deaf and that was the reason he got the job in the first place.
It was assumed that a deaf bookkeeper would not hear anything that he might have to testify about in court.
When the mobster goes to confront the bookkeeper about his missing $10 million, he brings along his attorney, who knows sign language.
The Godfather tells the lawyer 'Ask him where the $10 million bucks he embezzled from me is'.
The attorney, using sign language, asks the bookkeeper: "Where is the money?"
The bookkeeper signs back: 'I don't know what you are talking about'.
The attorney tells the gangster: 'He says he doesn't know what you're talking about'.
The gangster pulls out a pistol, puts it to the bookkeeper's temple and says, 'Ask him again!'
The attorney signs to the bookkeeper: 'He'll kill you if you don't tell him!'
The bookkeeper signs back: 'OK! OK! You win! The money is in a brown briefcase, buried behind the shed in my cousin Enzo's backyard in Queens!'
The Godfather asks the attorney: 'Well, what'd he say?'
The attorney replies 'He says you don't have the guts to pull the trigger.'