You might have silenced the sound in Windows Media Player or on your system. Follow the guidelines below to ensure that the volume is audible.
For Windows Media Player:
1. Run the program.
2. Check the volume icon. If there is a red mark on the icon, click it to remove the mark and make sure that the audio slider is not at zero.
For the system sound:
3. Click the speaker icon on your taskbar.
4. If the speaker icon has a cross near it, then click on the icon and make sure that the audio slider is not at zero.
For the Volume mixer:
5. Right-click the speaker icon on your taskbar.
6. Choose Open Volume mixer from the pop-up menu.
7. Here you can configure the volume for individual apps. Make sure that the Windows Media Player slider is not at zero.
Now both system and player sounds should be unmuted. If this does not help, then there has to be another reason for the volume not working.