(from our FAQ)
Can I use the DPS Plug-In with the AirPort Express?
Yes. However, you must first purchase a program called AirFoil by Rouge Amoeba. http://www.rogueamoeba.com/airfoil/mac/ This program will hijack all audio from iTunes and transmit it to your AirFoil or Apple TV. However, there is a considerable delay (about 1 second) when using this method. This is not an issue when using a playlist but switching tracks can be cumbersome. Also, video playing in the iTunes window will not be in sync with audio playing through the AirPort Express.
Setup is simple:
1) Install the AirFoil software
2) Set the “application” menu to “iTunes”
3) If your network is setup properly you should see your AirPort Express. Select it by clicking on the speaker icon. It will display “Active”.
4) You AirPort Express should be connected via the 1/8” jack to your sound system. Be sure your sound system or speakers are on.
5) Play audio in iTunes. You will notice a delay from when you hit “play” to when you hear sound.
6) Enable the DPS Plug-In. The same delay will occur. You will now hear the DPS technology optimizing your sound system!