Xaudio is an Xbox-only API designed for optimal digital signal processing.
XACT also includes X3DAudio, a spatialization helper library available on both platforms, Windows and the Xbox.
It was designed to allow sound designers and composers for the original Xbox console to have access and control of the powerful MCPx audio chip in the console, which previously could only be accessed through program code, via Xbox extensions to DirectSound.
XACT was the first widely available game audio tool that allowed a sound designer to run a tool connected to a running game and modify the sounds in real time, as the game was running, and support easy streaming of audio data from the console hard drive.
Later, under the XNA initiative, it was re-written to work on both Windows and Xbox and renamed the "Cross-platform Audio Creation Tool" and included as part of the DirectX SDK.