1. An apparatus, comprising: a first acoustic speaker; a first set of one or more acoustic microphones; and a dental appliance, wherein: the first acoustic speaker and the first set of one or more acoustic microphones are attached to the dental appliance so that, when the dental appliance is placed into a user's mouth: the first acoustic speaker has a position that causes acoustic waves generated by the first acoustic speaker to enter the user's vocal tract; and the first set of one or more acoustic microphones has a position that causes acoustic waves reflected from the vocal tract to impinge onto at least one microphone of the first set.