If you have removed the cover in the past, you may have ripped the speaker wires from the motherboard.
Be VERY careful when removing the back cover of this tablet, the speaker is screwed to the back panel, and the leads that run from the speaker to the motherboard are very thin and there is very little slack (-/+) about an inch, 1.5 inches.
My butterfingers slipped and tore the speaker wires from the motherboard. The only way to put them back on is to solder then back on with a soldering iron. The red wire goes to the positive marked spot next to the battery connector on the right.