This is not the first app that sets out to do this, but it is one of the simplest and coolest I've ever tested. How AmpMe, you connect multiple smartphones (iOS and / or Android) to create a kind of sound system with multiple speakers.
The first good news is that the app is completely free. The second is that it is very easy to connect the devices, just type a four-digit code informed on the device that will control the “party” and bring them together to do the sync by the audio itself being played.
On the other hand, for the time being AmpMe only works on the basis of SoundCloud (other services are planned for the future). Furthermore, in my tests, the synchronization was not always perfect, generating a delay minimum that gives the echo sensation.
Watch a promotional video of the app:
AmpMe is available on both the App Store and Google Play.