[knob]

I'd say the headphone socket is faulty, I've got a rockchip player with a similar spec to yours and it always cuts the speaker off when the headphone socket is used.
Updating the firmware is risky if you don't have the correct version for your player, just look through this forum and see how many people have bricked their players doing this.
If you have only just bought your player I'd suggest returning it to Amazon for a replacement.