This has been happening since my first iPhone. Usually it's my gps apps, randomly when I start navigation they won't have any sound. I try completely quitting the app and restarting, playing and stopping audio in other apps, quitting apps, etc. Nothing but restarting the phone fixes it. As you can imagine doing all this is incredibly dangerous when I'm driving and restarting is slow and I usually don't realize the problem until the first or second turn. Why is this happening and how do I fix it sort of constantly restarting my phone?