See the PortAudio website for further information http://www.portaudio.com
Read the PortAudio API Overview for a top-down view of the PortAudio API, its capabilities, functions and data structures. The documentation for PortAudio's main header file portaudio.h details the individual data types and functions that make up the API.
To get started writing code check out the tutorials on the PortAudio Wiki: http://www.portaudio.com/trac/wiki/TutorialDir/TutorialStart
This documentation covers the current API version: PortAudio V19, API version 2.0. API 2.0 differs in a number of ways from previous versions (most often encountered in PortAudio V18), please consult the enhancement proposals for details of what was added/changed for V19: http://www.portaudio.com/docs/proposals/index.html
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If you're interested in helping out with PortAudio development we're more than happy for you to be involved. Just drop by the PortAudio mailing list and ask how you can help.