Music Player Daemon (http://musicpd.org http://musicpd.org) would be one example. It operates as a OS daemon -- therefore without any UI per se. It is controll

Music Player Daemon (http://musicpd.org http://musicpd.org) would be one example. It operates as a OS daemon -- therefore without any UI per se. It is controlled it using a text-based protocol over TCP/IP. Usually, the user controls it via UI frontend that can be graphical, text-based or what have you. And you can have as many of those as you like (for instance one for xwindow, another for command line, third that shows you what is currently being played, fourth for album art and lyrics...)