*WARNING*: this is a *very old* project. Don't expect much.

p4sync
http://snarfed.org/synchronizing_mp3_playback
License: This project is placed in the public domain.

p4sync syncs mp3 players on multiple computers so that they are playing the same song at the same time. p4sync will be distributed as a plugin for many popular mp3 and media players - we initially plan to support Winamp and XMMS.

A few older projects have attempted to sync mp3 players on multiple computers, but they all fell victim to the same problem: network latency. These projects were effectively designed to yell "GO!" across the network to all connected hosts. This often results in unacceptable sync quality, because the human ear can detect sources that are out-of-sync as little as tens of milliseconds.

We plan to use a superior sync mechanism. Each host will use NTP to sync their system clocks. The p4sync server will record the time it started playing the current song. When a client connects to the server, the server will report the time at which it started playing the song, and the client will calculate the current offset into the song. We already have a running prototype, and it is capable of syncing such that humans cannot detect that there is more than one music source.

p4sync has a few high-level goals. They are, loosely in order of priority:
1) Flawless sync quality
2) Cross-platform and supports multiple mp3 players
3) Best practices (i.e. unit tests, logging, aspect-oriented programming)
4) Easy to install and run
5) Desirable features such as playlist and file propagation