From the documentation, you have to call release() on an Equalizer, MediaPlayer, Visualizer, etc for a graceful exit, or you will see this error when restarting the app. The only remedy then is to reboot, as previously mentioned in this thread.
This is where the Android application lifecycle makes things a little difficult, since apps are never supposed to exit (just pause and resume), unless absolutely required by the OS for memory reasons, or a reboot occurs. Your app onDestroy() method is called in both cases.
You can put the release() in onDestroy(), and that would satisfy the Android lifecycle for deployed apps. Your users would not see this error.
In development however there is a problem: IDEs like Eclipse (which is actually a framework for building IDEs, and not meant to be an IDE itself...) will kill the app process instead of sending it a destroy message. This violates the lifecycle and release() is not called.
This is also why you should never call System.exit(). It violates the lifecycle at the risk of ungraceful exits exactly like this.
So your process was exiting ungracefully. Only happens in development, not deployment. One remedy is to not use the device window in eclipse to stop processes. It is not a stop, but a kill.
Eclipse also kills (lifecycle violation) the process ungracefully when you Run an app project while there is already an instance running.
As the doctor said, if it hurts, don't do it: instead use the debugger which sends actual lifecycle messages to the app.