If you want audio from your computer, tablet, phone, or other device to play through your keyboard's speakers, you will need to use a stereo audio (TRS) cable to connect from the headphones jack of your audio device to the AUX IN jack of your keyboard.
The keyboard's AUX IN jack takes a 1/8" (3.5mm) plug, so the exact type of cable depends on what size and type of plug needs to go into the headphone jack of the audio device.
A computer normally has a 1/8" (3.5mm) jack for headphones, so you can use a basic 1/8" (3.5mm) stereo cable, commonly called an "auxiliary cable," to play your computer's audio through your keyboard's speakers.
But a tablet or phone might have a 3/32" (2.5mm) jack for connecting headphones-- and in some cases, the jack is designed to take a TRRS plug rather than a TRS plug so that you can use a headset (that is, headphones plus a mic for speaking into). So if you plan to play audio from a tablet or phone through your keyboard's speakers. first you'll need to refer to the owner's manual for your tablet or phone to see what kind of plug needs to go into the jack. Then you'll need to either get a special cable (3.5mm TRS to 2.5mm TRS, or 3.5mm TRS to 2.5mm TRRS, or possibly 3.5mm TRS to 3.5mm TRRS) or else use a standard auxiliary cable (3.5mm TRS to 3.5mm TRS) with an appropriate adapter.
The keyboard has an AuxInVol (Auxiliary In Volume) function to control the volume level of the auxiliary audio on the keyboard's end, but you will also need to set the audio volume as desired on your computer, tablet, phone, stereo player, or whatever type of audio device you're using. I would recommend setting the keyboard's AuxInVol to 100 (which is actually the default setting) and adjusting the volume on the audio device to be as loud as you'd like it to be under normal situations. Then you can use the AuxInVol function to turn down the volume of the auxiliary audio coming out of the keyboard's speakers if desired.
