Why won't music in my apps play?

I just got a new iPhone 4 and sometimes the music and sounds in my apps don't play or are quiet until I close and open the apps a few times. Music in the music player plays fine. Is there a way to fix this?

Added (1). It's obviously not the volume. The volume is all the way up when this happens.

Added (2). It's obviously not the volume. The volume is all the way up when this happens.

Turn up the volume

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