Some system units have the headphone jack wired with its Left and Right channels reversed, so you hear Left output in your right ear and vice versa. The line-out jack at the back of the unit works as expected.
To find the active audio input jack on the back of your system, plug in a sound source. Run audiocontrol(1) and select Record. On the Record panel, turn the Record Volume and Monitor Volume sliders up so you can hear the output. Then select Line In and, secondly, the Internal CD to find which audio input port produces sound. If the Internal CD button does not appear on the audiocontrol Record panel, use the Line In selection for the audio input. Use the Microphone button to select the microphone jack on the keyboard.
The quality of sound is better when using an external microphone and speakers, not the ones built into the keyboard.