Multizone Playback
From JRiverWiki
Player > Playback Options
Media Center can control multiple soundcards at a time. Each zone has its own playback and DSP settings. You can quickly flip between zones.
This is ideal for DJ's that preview on headphones while playing for the audience, or people with a few rooms in their house. Each zone has its own Playing Now window and each can be playing at the same time. Play completely different playlists at the same time!
And even if you have only one soundcard, you can still use multizone playback. Use it to manage several playlists during a party (one for rock, another for jazz) and just go back and forth between them while keeping the current position for each one.
