Can the RGB lights be turned off?

Yes, the lights can be turned off. Short press the button can change the light mode. Short press the button three times to turn off the lights.

What should I do when I plugged it into my computer it only show the lights and not play any audio?

Please check if the 3.5mm AUX cable is pluged correctly to headphone jack and make sure that "Headset Realtek Audio" is selected as your computer playback device.

How long of the cable between two speakers?

The length of the cable between two speakers is 3.28FT(1M). The whole cable length is 4.76FT(1.45M).

Is this computer speaker Bluetooth supported?

Yes, the Bobtot computer speakers are Bluetooth 5.0, double click the button to switch the mode from AUX to Bluetooth mode. Using your device to search the Bluetooth name "Bobtot A10".