If the game sound unexpectedly went out, please attempt the following:
1) Make sure your physical mute switch is not off. Try toggling the mute switch off and on to make sure.
2) Open Zombie Gunship, tap the gear icon in the upper right hand corner, and tap the Audio off and on.
3) Reboot your device by turning it off and back on again.
4) On iPad in the General Settings you can set the switch to be lock rotation or mute. We have reports that toggling between those settings fixes the missing sound problems.