AmpMe!

Listen to your music on multiple devices in sync share your music with friendsBuild a free multiroom audio systemConnect up to 16 devices simultaneouslyUse SoundSeeder with every existing Wi-Fi networkSupports Wi-Fi direct and Androids built-in portable HotspotFull-featured Music Player with Media Library and EqualizerDatabase with more than 4500 Online Radio StationsExperimental Google Music supportSoundSeeder is supported by Android 4.1+Speaker App available for Android 2.2+Windows and Linux Speaker Application (Raspberry Pi support) »

AirPhones is an app that sends perfectly synced audio from your computer to your iPhone or iPad with no latency. It's a next-generation, solution to wireless headphones.

play music on multiple phones at the same time.