1. Try turning the tablet off completely and then back on. This may resolve the problem.
2. Check that nothing is muting it (in any app that is running).
3. Try starting the phone in "Safe Mode" to see if the sound works then. If yes, you have a problem app.
4. Try using headphones and see if you get sound that way. If yes, it could be the physical speaker. If this is the case, the speaker would need to be replaced.
NOTE: Do not do any physical work on the device, or have someone else do it for you, if the tablet is still under warranty. If that is the case, contact the manufacturer about a repair or replacement.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.