This has been happening ever since I can remember and just now thought I'd post about it. If I receive a phone call during game play and take it, upon return to the game all sound fx's are gone except for the background music. Only way to fix is to close the game completely with a double tap of the home button and swipe up to close the game, a simple exit and return does not work. Not a huge issue really but just thought you'd like to know.
