How to stream music from a Mac to an iOS device?
Airfoil from Rogue Amoeba can stream audio from the Mac to many other devices, including iOS. It will capture any audio running on your system, either from a specific input (e.g. Mic), application, or the entire system audio.




  Airfoil Speakers Touch 
  
  Airfoil Speakers Touch runs on iPhones, iPods
  Touch, and iPads running iOS 4.0 or higher. Use it to send audio to
  your iOS device when connected to your local network! Airfoil Speakers
  Touch works exclusively with Airfoil for Mac and Airfoil for Windows.