You can do this by going into any game and navigating to the bottom of your game.  To adjust the Volume of your game, you can click or tap on the speaker symbol. This will bring up a level to select how loud or soft you want the background music. And clicking or tapping on it until it turns red will mute any music.

You can also select the gear symbol above the speaker symbol and turn on and off other sound settings.

   
Or for Mobile:
Select the Gear Icon 

Then you can toggle the Sound from On to Off, or vice versa.  You can also adjust the volume using your phone's volume settings.