What is the purpose of the hyoid bone?
The hyoid bone (lingual bone) (Latin os hyoideum) is a horseshoe shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage.
It is the only bone in the human skeleton not articulated to any other bone. It is held in place by thyroid ligaments. The hyoid bone provides attachment to the muscles of the floor of the mouth and the tongue above, the larynx below, and the epiglottis and pharynx behind.
Its name is derived from the Greek word hyoeides meaning "shaped like the letter upsilon" (υ).
Specifically, as per inquiry:
The human hyoid bone allows for the production of a wide range of sounds that other animals cannot produce. It allows a broader range of tongue, pharyngeal and laryngeal movements by bracing these structures alongside each other in order to produce variation.
Paraphrased from Wikipedia
I don't think you die when your hyoid bone is broken. I think that if you're dead, a broken hyoid is strongly indicative of strangelation. So, if you're dead and if you have a broken hyoid, you were probably strangled. The hyoid breaking just occurs, but I think that it's suffocation from lack of air that will kill you...not the hyoid bone itself.
The action of the omohyoid is to depress the hyoid. Its name derives from the Greek "omos" meaning shoulder, giving one of its attachments, and "hyoid", giving the other attachment - the hyoid bone. The function of the hyoid seems to be in voice variations. All mammals have them and there are arguments if primitive hominids used them as we do.