Theremin - Undead Mac OS X MPD Client. Theremin is a MPD client for OS X. It allows you to connect to your MPD (Music Player Daemon) on another machine on your network to control the music it's playing. You can also manage playlists, reorganize the songs in your playlist, and browse your music library. It also supports the media key on your Mac keyboard (Play/Pause, Previous song, Next song).