iOS Universal Music
With BlueTunes you can listen to your music and use other iPhones and iPads as loudspeakers over Bluetooth and WiFi.
The procedure is simple: On one iPhone or iPad you choose "Serve Music" and listen to your music as you would do with the iPod app. On all other iPhones and iPads, where BlueTunes is running, you see serving iOS devices in a list. By tapping on one of them you are immediately using the current device as loudspeaker.