I tend to think that we think in actions and consequences. It just so happens that speech is an action which has a very tight loop for direct consequences which other parts of the brain can make use of (sound waves). When you're angry you imagine punching someone, or if you're happy you imagine jumping around ecstatically, etc. The tight coupling between sound an movement means that the agent in the brain for most people just uses the auditory representation since it is more salient (grabs the attention better) than the motor movements of the mouth/tongue/vocal cords. When learning a new language and trying to figure out how to produce a new phoneme if all you think about is the sound that comes out rather than what you are actually doing with your speech motor system you will have a much harder time learning. Consider a deaf person whispering or a 'truly' deaf person (who can't hear low frequencies via bone conduction) speaking. They can do it, but they must be thinking in terms of the movement of their mouth and vocal cords, not in terms of the 'sound' they make.
