AHI is (still!) the de facto standard audio subsystem for AmigaOS, MorphOS and AROS. It handles sound card arbitration and sharing, real-time audio mixing and virtual audio channel management.
AHI was written by Martin Blom more than ten years ago, while he was still an inexperienced student and didn't even know C, but had plenty of spare time that could be spent coding. The situation is quite different today, especially regarding that spare time part, but AHI is still being maintained and even improved upon once in a while. Over the years, lots of people have contributed to the project.
AHI is released under a mixed license, where the executable programs are covered by the GNU GPL license and the device/library is LGPL'ed, making it possible for non-free software to use the system without restrictions.
The GPL and LGPL give everyone the right to freely modify, enhance, hack and rebuild AHI, and also to redistribute the result to others. You are encouraged to do so!