The speakers in this system are controlled by relays, and simplest possibility is that the changeover connections on the rear of the headphone jack socket which control the relays are maybe heavily oxidised and haven't made properly again when the headphone jack has been removed, try rapidly pulling the headphone jack plug out and in a few times as it might just trigger the contacts into action. If this doesnt cure the problem then this part of the circuitry will have to be checked by internal investigation.