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You can use AirFoil to do this.

http://rogueamoeba.com/airfoil/mac/

You install that and install AirFoil for the iPhone from the app store. The iPhone app is free and the Mac version comes with a free trial, cost $25 afterwards.

Thanks a lot! This is exactly what I needed.

Everything works fine, but there is a noticeable lag between Mainstage and Airfoil. About 3 seconds when I play something through Mainstage until it reaches my phone and earphones.

With iTunes there is no lag, it's virtually instant play.

Any idea how to fix the latency? The WiFi signal is strong so I doubt it's that.
 
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