Headphone parties, or silent discos, seldom translate well to mobile devices -- the likelihood that every listener has the same songs is rather slim. If Apple implements a newly granted patent, however, it could be easy to start those private gigs. The technique shares the tempo of a master track with other devices invited to a party; those gadgets automatically pick similarly paced tunes and sync their playback. Participants could be part of an ad hoc local network, but the approach would also work when people are miles apart. Apple even proposes a social networking element that lets aspiring DJs share avatars and other identifiers. While there's no guarantees that the patent will reach shipping products, we wouldn't be surprised to see iPhone owners grooving in unison at some point in the future.