I have a Yamaha PSR300. No sound from the internal speakers but all else works. I have traced the fault to both speakers have gone open circuit! This appears to be because the braid from the solder tags to the speaker cone , which appear to be copper plated cotton, have corroded near the sealant used and the copper no longer conducts the signal. Replacement speakers is the only fix , or use an external speaker set up plugged into the headphones socket.