Yamaha PSR-E453 inbuilt speakers not working when there is no output speaker connection

I'm working on a Yamaha PSR-E453 whereby, the inbuilt speakers are not giving any sound output but when I connect external speakers, the sound works fine. I've checked the output port fir connecting ths speakers to see if there's any jack stuck therein, however there is none. The sound from the inbuilt speakers ceased abruptly while being played. What coukd be the issue