[ozzfjlzv] Synchronized music players

Create a system which two or more people can play and hear the same song, or playlist, simultaneously on separate portable music players (e.g., smartphones).  The intended application is silent rave or silent dance party, especially for partner dancing where both dancers should be hearing the music synchronized within a quarter of a beat (sixteenth note), approximately 100 milliseconds.  (However, a greater challenge would be accurately synchronized to much higher precision.)

Challenges: NTP over wireless latencies.  Alternatively, perhaps a local way of synchronizing, e.g., tapping the phones together or NFC.  The songs have to play on both players at exactly the same speed, i.e., within 100 milliseconds throughout the whole song.  Normally, a single music playing device can ignore the latency between when the song data file is read from media and when the sound waves get produced by the speaker cone.  However, to produce synchronized sound, this latency matters, and managing it might be tricky if two devices have different latencies.  What if one person listens using a Bluetooth headphone while the other person uses wired headphone?  I suspect the Bluetooth stack induces greater latency.