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!
I think Seedio and Seedio Free do what you want.
back in 2013, I was one of the co-founders of Audibly, wireless synchronized surround sound for iOS. I was kind of late to see this while googling something completely unrelated to the question, but I was like Hey, I built what he needs! This'll take a sec.
https://itunes.apple.com/us/app/audibly-make-your-music-heard./id882170258?ls=1&mt=8
Another reason why jail breaking is so amazing :) , jailbreak it and install airplayspeaker and there u go :) or there zillions of apps that do that to though I don't remember there names but airplayspeaker built in so why not take advantage of that
AmpMe on iTunes is built exactly for that reason. Check their website.
From what i can see, it supports both Android & iOS. Enjoy the party & picnic.
For Android users, here is the Google Play Store link.
