The Android OS may be experiencing a problem where you are unable to hear the sound of any video that is being played. To avoid this problem, simply restart the device, and the sound will play again. This step should be completed before beginning any other training so that the momentary glitch can be resolved flawlessly. Other difficulties, in addition to the sound issue, can be fixed by following this procedure.

If you aren't utilizing any third-party software, this can be considered a final resort. You must back up all of your data and contacts, or you will lose everything. Select Settings > Backup & Reset > Factory Data Reset > Reset Phone from the drop-down menu.