1. An apparatus, comprising: a first acoustic speaker; a first set of one or more first acoustic microphones; and a dental appliance, wherein: the first acoustic speaker and the first set of one or more first 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 first acoustic microphones has a position that causes acoustic waves reflected from the vocal tract to impinge onto at least one of the first acoustic microphones; a second acoustic speaker configured to generate second acoustic waves; a second acoustic microphone; and a headset having an earpiece and a boom arm that is movable with respect to the earpiece, wherein both the second acoustic speaker and the second acoustic microphone are mounted on the boom arm, wherein: when the headset is placed onto the user's head, the boom arm is configurable for: the second acoustic speaker to have a position that causes the second acoustic waves to reflect off the user's lips; and the second acoustic microphone to have a position that causes reflected second acoustic waves to impinge onto the second acoustic microphone.