把沒連接的2環耳機線（或電腦喇叭線，它們是一樣的）插入耳機孔即可，對鍵盤無害，實測有效
If your keyboard has speakers you wish to disable while monitoring to the line outputs, then putting an unconnected jack plug in the headphone socket is a harmless means of muting them.
https://www.soundonsound.com/sound-advice/q-it-safe-leave-dummy-jack-plugged-my-headphone-socket?amp
