SoLoud, like everything else, stands on the shoulders of giants; however, care has been taken to only incorporate source code that is under liberal licenses, namely ZLib/LibPNG, CC0 or public domain, or similar, like WTFPL or Unlicense, where you don't need to include mention of the code in your documentation or splash screens or any such nonsense.
Any patches submitted to SoLoud must agree to be under compatible licenses.
Please check our license page for more detail.