If you quit the app and restart it, the sound will return.
To truly quit an app :
Press the home button to leave the app.
Double click the home button, this will reveal your recent apps,
The last one you used will be on the left. Press and hold the app's icon
And a 'x' will appear at the corner of the icon. Press the x and the app
Click the home button again, then restart the app in question.
Your sound should now be working.
I don't know why the sound stops, this happens to me on all my
IOS devices, iPhone, iPad, iPod.
Hoping some future update fixes the problem, but until then my