The way I do it also allows you to save it, so every-time you turn on the keyboard the speakers are muted:
1.-Press Global and go to Audio Setup
2.-Touch Met/MP3/Speaker tab, then check speaker square.
3.-Touch Upper Right top corner ands Save Global Set up.
This way, you do not have to plug nothing into the headphone and I think is better for the keyboard and you too.
I hope it helps you.
------------------
mdorantes
_________________________
mdorantes