A biometric-sensor assembly, e.g., for acoustic reflectometry of the vocal tract. In one embodiment, the sensor assembly includes a dental appliance for in-mouth mounting, an acoustic sensor attached to the dental appliance, and an optional headset. The dental appliance enables secure placement of the acoustic sensor in the mouth of the user, e.g., for tracking movements of the tongue and/or other internal articulators of the vocal tract. A boom arm of the headset can be used for holding an additional acoustic sensor and/or a miniature video camera, e.g., for tracking movements of the lips.