iPhone sound problem! Help please

I bought a brand new iphone 4s yesterday, and downloaded the awesome new game Jetpack Joyride. But after playing it for a few hours, the sound effects stopped working. The music continues on, but there's no more sound. I checked if the sound was turned on in the game options and it was.

So i reinstalled the game and everything was all good, until somebody calls me. After we hung up, the sound effects dissapeared again!! My other games all have perfect sound, only Jetpack Joyride has issues.

I know my problem seems pretty insignificant, but the main reason I purchased an iphone was for the portable gaming. And having no sound effects on my favorite game really throws me off.

So please, if anyone knows how to fix this problem it'd be much appreciated. Thanks!!