If the phone/tablet is set to silent then Word Chums will not play sounds. Make sure you dont see the red dot on the switch above the volume buttons on your phone. Many apps ignore the mute switch so you may still be hearing things in other apps with the mute switch on.
Assuming neither of these turned the sound off its possible something got out of synch. The first thing to try is to make sure Word Chums is completely shut down and then restart it. To shut it down go to any home screen on your device (a screen where you see all your app icons). Double tap the home button on the front of device. Then:
In iOS 7
You can swipe left and right until you see Word Chums. Then swipe UP on the Word Chums "card".
In iOS 5 and 6:
You should see a row of apps at bottom of screen. Assuming Word Chums is one of those...tap and hold Word Chums until the apps wiggle and you see a minus sign. Then tap on the minus sign on Word Chums to shut it down. Now try to open Word Chums again and see if it works.