3rd ed Speak with Dead says: You can cast this spell on a corpse that has been deceased for any amount of time, but the body must be mostly intact to be able to respond. A damaged corpse may be able to give partial answers or partially correct answers, but it must at least have a mouth in order to speak at all.
4th ed Speak with Dead says: You ask the corpse of an intelligent creature questions and receive answers.
So ask your GM about how the ritual works in their games and if the body needs a mouth to talk. If it does need a mouth, then remove the jawbone (it's traditional).