[knob]

Well it's always worth a try rechecking the soldering first.
I have found however that if you have the wrong firmware loaded on a Rockchip player it can have quite an effect on the hardware too.

From what i can gather this player has never worked correctly so it may even have had the wrong firmware already loaded on it.

Loading the wrong firmware on my player I have had the following happen,
loss of screen, buttons mixed up, distorted sound, lines on screen and corrupted flash memory, another user has reported loss of his loud speaker so it's a risky business.

Still if the player doesn't properly it worth looking at everthing.
hope you can get it back, it's a nice looking player.