Be sure you are using the charger that was provided from the manufacturer. Generic chargers tend not to charge the device properly and may kill the battery cells. If you are using the appropriate charger, and your nextbook is still not holding a charge, it is likely that your battery is dead and you will need to buy a replacement.

The speaker could be filled with dirt which could distort or block the sound entirely. Take the back cover off, remove the speaker, and blow on it. If that doesn't seem to clear the dirt out, try using compressed air to remove any extra debris. If after cleaning out the debris, the speaker still doesn't produce sound, the speaker is probably faulty. It will need to be replaced.

The camera lens could be dirty and may need to be cleaned. Clean the lens with a cloth to remove smudges or dirt. If that doesn't work, take the back cover off and clean the inside of the lens and the actual camera itself. Be careful doing this so you don't scratch the lens. If the images are still coming out blurry, you may need to replace the camera.