If I had seen this thread from the beginning, I would have bet correctly that the problem was at the sound amplifier. What happened was, that the amplifier for some reason got fried, so some electric current was passing at the speakers at the moment you tried to start it.
This may happened by hardware failure, which is not so common. The most common way that an amplifier dies, is when you use the internal speakers at very high volume or at the maximum that the amplifier can produce. This can damage the speakers and then if the speaker's coil get a short circuit, then the next part that it will be damaged will be the sound amplifier. Also some times this can happen if you plug and unplug some external devices from the audio input or output sockets and you forget to put the master volume at zero, so you hear some fast scratch crackling noises.I guess at the Yamaha center they have checked the speakers too.
Anyway, don't worry, this is a common fault at car stereo and home hifi devices and it is fully repairable, so afterwards the device becomes as before.
Just take care about the above situations and you will be fine.Let's wish from now and on, to enjoy your cute s970 keyboard without problems for many many years as with your old one.