Audio-3D.com has Audio-3D Player 7.1 (iTunes Link) for the iPhone and iPod Touch in the iTunes App Store for FREE (regularly $5.99) for a limited time.

The first 7.1 audio player for smartphones in the world. It features 44KHZ / 24 bits output, 2.1 HIFI emulator with 360° rotation, 7.1 Home-Cinema emulator, and Headphones optimisation.

The 7.1 emulator allows you to listen to your music as if you were in a room listening to a home-cinema.
With the 3 modes available (normal, dynamic and spectacle), you can choose the best to fit with your style. And if it is not boomy enough for you, a bass boost option is here for you.

The 2.1 emulator allows you to reproduce the sound of an HiFi system. You are at the heart of the audio scene and you take control of the sound directivity : Move away or move closer, and spin the sound all around you with accurate precision. 
This emulator has also has a custom made equalization system adapted to your headphones witch optimize perfectly the sound quality. Get a HQ HiFi sound on your iDevice!