The build system and framework for installing and managing plugins is:

Copyright © 2001-2016 Audacious developers

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:

    1. Redistributions of source code must retain the above copyright notice,
       this list of conditions, and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright notice,
       this list of conditions, and the following disclaimer in the
       documentation provided with the distribution.

    This software is provided “as is” and without any warranty, express or
    implied.  In no event shall the authors be liable for any damages arising
    from the use of this software.

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The plugins themselves are distributed under their own distribution terms.
Read the source code for individual copyright holders and licensing details.