Hi Jin Yang
Firstly please understand that I am not a techo. Gary and Michael are very knowledgeable about these things and they offer very good advice. However, if I had your problem, this how I would be thinking . . (as a layman).
The power is getting through to light up everything. So the batteries and/or power adaptor are okay. The speakers are not responding and could mean that the fault is a computer glitch. The speakers are not getting the right signal. The circuit is broken. So here is a thought to clear the glitch (what I woud do) . . I would perform a factory reset thinking the keyboards inbuilt computer may then act normally and send the correct signal through the circuit to the speakers. That is just my layman practical brain thinking
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If that fails . .
I would ask, do the speakers emit any sound at all? When turning the knob is there any crackle or hum? If so the potentiometer may have some grit or moisture in it. Try a squirt of dewatering fluid. (WD40 or CRC).
These are just ideas and not necessarily cures. Not even sure if the WD40 is a good idea or not . . GARY?
What is the history of this unit? Did you buy it recently? Has it been in storage?
If so in a dry atmosphere? Did the fault happen suddenly? Has it been dropped or damaged?
Is it worth fixing? Probably not. It is not an expensive unit. As Gary and Michael have already said . . Maybe just get yourself a newer model and give this one the flick.
Ray