While using line out, the built-in speakers still produce sound. How can I silence these speakers?

The CTK-7000, CTK-7200, WK-7500, and WK-7600 are the only current (as of 6/4/14) musical instrument models that have the ability to silence the built-in speakers while still outputting sound to line out. On these keyboards, there is an option called "Speaker" under the General group in the Function menu. Turn this option off to silence the speakers. Please see page E-133 of the CTK-7000/WK-7500 manual or CTK-7200/WK-7600 manual for more information