The motor theory was born because it was found that the acoustic speech signal is ambiguous. The same sound, eg /d/, can be cued by different acoustic features. On the other hand, the speech gesture that produces the sound /d/, it was suggested, is not ambiguous, it always involves placement of the tip of the tongue on the roof of the mouth. Therefore we must perceive speech by accessing the invariant motor gestures that produce speech.
There is one thing that never made sense to me though. If the acoustic signal is ambiguous how does the motor system know which motor gesture to access? As far as I know, no motor theorist has proposed a solution to this problem. Am I missing something?