Default I'm looking for an app or have an app idea for external control of an android.

Hi XDA,

Since i'm working and barely have time to look at my phone whilst i'm working, i would like to have a way to control music playback (such as next song, play/pause, louder, quieter) without taking it out of its etui. I'm thinking it would be possible to control the phone via tapping on it, while it's in the pocket.
Is there any app out there, where it would be possible to do so?
If not, i think it would be easy to make such an app.. using either the microphone to pick up the tapping noise or the accelerometer in some way.

Thanks,