Yeah, that happens sometimes. I think it's in the coding of the iPad, in that it turns off sound in one app in preparation of needing it for another.
Most of the time, there is an easy "fix." If your sound disappears, double click the Home button (or do the four-finger swipe up gesture) to reveal the multi-task bar. Swipe to the right to bring the left side into view. You should see the app that was playing the music there on the right. In the center of that view is the Play button. Press that and your radio should start. Then, you can tap on the screen again to go back to wherever you went that caused the sound to stop.
Again, I believe this is a function of the iOS. But, this method should work. Please, let us know how it goes.
Marilyn