I heard FMOD is one of most popular audio engine in desktop apps such as games. I have read its documentation, but I can't feel any benefits of them than Apple's API like Audio Unit. It has many of powerful features, but it does not look usable on mobile device like iPhone because processing power is too weak.
However maybe this caused by my shallow knowledge and experience. What feature (or attribute) can I count benefit of FMOD than Apple's API? (not only FMOD, but also other sound systems if you know)
Full disclosure: I am a programmer working at Firelight Technologies, the makers of FMOD.
Firstly I'd encourage you to try FMOD out, it's freely downloadable, have a look through the examples and the documentation. Depending on what your application or game requires will greatly affect the benefits you will receive from using FMOD.
With FMOD you'll have features like our virtual voice system which makes efficient use of limited resources, there is a whole swag of DSP effects you can apply to your sound, we provide sample accurate sound playback for stitching and syncing. Along with all the standard volume, pitch, 3D spatialization, rolloff, etc features you would expect. Not to mention we take full advantage of iOS hardware / software decoding capabilities, and we handle interruptions like alarms and calls automatically.
If you want to step up to a more sound designer, data driven implementation we have FMOD Designer. Here you can create complex multi-sound parameter driven events, randomized ambiances and controllable music systems all in a graphical tool with minimal programming required to implement.
There are plenty of benefits to using FMOD, or just audio middleware in general. The best advice I can give is try them out for your needs. If you are interested in learning more about FMOD, try our forums: http://www.fmod.org/forum