OutLoud Social Jukebox is an app that lets your friends control the music at your party in real-time. With OutLoud, you'll always play music that best fits the crowd and the moment. You set up a playlist and have your friends add and vote for songs; the song with the most votes is played next.
Switchboard Music lets you listen to music together. Drop in through the intercom or keep voice/video chats open indefinitely for totally spontaneous conversation. The music volume adjusts automatically when someone speaks. You can listen to your own music or synchronize with your friends.