Check to see if one of your two speakers are broken by putting your ear up close to the speaker openings. If sound is only coming out of one of them use this guide to replace the broken speaker.
Sometimes the rear case can be very tight. Make sure you have removed the two screws described in this guide.. Once you have those removed try using more than one plastic opening tool to pry the rear case off. If this is the first time taking your tablet apart it may be very tight from manufacturing.