Audio Units (AU) are a system-level plug-in architecture provided by Core Audio in Mac OS X developed by Apple Computer. Audio Units are a set of application programming interface services provided by the operating system to generate, process, receive, or otherwise manipulate streams of audio in near-real-time with minimal latency. It may be thought of as Apple's architectural equivalent to the other popular plug-in format, Steinberg's VST.
Kontakt Player is a FREE, the most fully featured sample player. It has the same engine as famous Native Instruments Kontakt 4 and hence supports all it's native formats. Kontakt Player also provides reasonable control over the sound.