OpenAL is already built-in to iOS. You don't have to worry about licensing or legal issues.
To use it:
Link your app against the OpenAL framework. In Xcode, find your app's target, select the "Build Phases" tab, open up "Link Binary with Libraries", press the + button, and add OpenAL.framework.
In your code, import the headers like this:
#import <OpenAL/al.h>