Ar you playing on an Android device that is running version 2.2?
If so, your sound has been disabled due to technical reasons. You are able to hear sound by playing the game in your browser which you can find links to via http://www.spacetimestudios.com.
You are able to check your version by tapping "Settings":
Next tap "About Device":
Your version number will be displayed and the number must be greater than 2.2: