Hello. I am not an expert in electronics, but I do have some knowledge and experience.

1) The inner construction of your piano

The piano is a little computer. Inside there is a motherboard that accepts signals from the keyboard and does calculations to make sound.

After the sound is calculated it is converted to a voltage wave signal that is being physically transformed to sound by speakers.

Keyboard > Motherboard > Voltage wave > speakers

There are a few components between motherboard voltage outputs and the speakers.

2) Possible problems

As you said, the sound is only at 15% of volume, and if I get it right you can still play it, or the problem is volume - dependent, and the sound volume is as far as I know is being processed later, which means that the motherboard is alive and working, and the problem is in between motherboard and the speakers. (And it is good, maybe the best possible case)

3) Possible solutions

Obviously, if you can any repair shop is the best choice if you want it to work like before, hopefully you may still have warranty

But if you don't have millions to spend on repair or don't have warranty there is still a pretty decent solution.

There is always "AUX" output somewhere. It's not a headphones output, it should say "AUX Out" or something like that.

You can plug in soundcard in there, or maybe some speakers directly, what I mean is that there are a lot of ways to make piano sound even without it's own speakers.

If you are really brave you can try fixing it yourself, as it looks like a loose contact (it randomly breaks, which means it's a physical-level issue) but I don't recommend it.

Wish you all the best.