iPhone app that allows simultaneous playing of music to multiple devices The idea is to use each iPhone as a speaker for one track of music. 

Picture it: you are in the park, having a picnic and you would like your music to be played louder. Why not sync all the iPhones to play the same song at the same time?

Does such an app exist?
Yes. You can use Airfoil, to stream music from your Mac using Airfoil for Mac to any iOS device with the Airfoil Speakers Touch app (free). It is as simple as turning on a speaker system, one button and you have the streaming audio from your Mac on your iOS device. I would highly recommend this!