PortAudio 19 SVN revision 1920 beta (Feb 2014)

PortAudio Portable Real-Time Audio Library 
Copyright (c) 1999-2011 Ross Bencina and Phil Burk

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

The text above constitutes the entire PortAudio license; however, the PortAudio community also makes the following non-binding requests:

Any person wishing to distribute modifications to the Software is requested to send the modifications to the original developer so that they can be incorporated into the canonical version. It is also requested that these non-binding requests be included along with the license above.
Plain English Interpretation of the License

The following is a plain English interpretation of the license. This interpretation is not part of the license and has no legal significance. To understand the full legal implications of the license you should consult the license itself.
You can use PortAudio for free in your projects or applications, even commercial applications.
You do not have to make your own source available as open-source code just because you used PortAudio.
Do not take our copyright information out of the PortAudio source code.
If you fix a bug in PortAudio, please send us the fix.
You cannot sue us if your program fails because of PortAudio.
We are not lawyers. But the general consensus is that this license is compatible with the GNU General Public License. In other words, PortAudio can be included in a GNU project without violating the GNU license.

In terms of legal compatibility, the PortAudio licence is now a plain MIT licence, interested readers could chase up general discussions of MIT vs GPL for more information. http://opensource.org/licenses/mit-license.php