1. A method of operating an optical motion-capture system in low-light situations, the method including: monitoring at least two contact microphones coupled to an optical motion-capture system, wherein the contact microphones are in contact with a surface that a user touches; and in response to detection of audio signals generated by contact of an object with the surface that the user touches, tracking a position of the object by: measuring a time difference of arrival (TDOA) of the audio signals at the contact microphones and mapping surface locations that satisfy the TDOA; analyzing at least one image, captured by a camera of the optical motion-capture system, of the object in contact with the surface; and using the image analysis to select among the mapped TDOA surface locations as a surface location of the contact.