lets let someone who already did the research tell us the final results…
I give you all OpenGameArt.org.
and their FAQ about using their content in your closed source games:
What do the licenses mean? I’m a commercial (closed-source) game developer. Can I use this art?
It depends on the license(s) the art is released under. Technicallyall of the art on this site is legal for use in commercial projects – however, some of the licenses require you to distribute the source code of your entire project for free, and allow others to distribute the source for free as well. Here is a quick overview of the licenses and what they mean for commercial, closed-source developers. Disclaimer: I am not a lawyer, and this is not legal advice. Read the licenses carefully and consult your legal department before including any of this art in non-open-source or commercial software.
Creative Commons Attribution-Share Alike 3.0
This license requires you to release the source your entire project under the same license or one with similar terms, such as the GNU GPL. If you’re trying to sell a game, this is probably something you want to avoid, as you will be required to distribute the source code, and your users will be allowed to distribute it as well.
Creative Commons Attribution 3.0
This license requires you to attribute the author of the content in the way that they specify. Provided the author is properly credited, it is generally safe to use this content in a commercial work.
GNU GPL 3
This license, like Creative Commons Attribution-Share Alike 3.0 above, requires you to release your entire project under the same license. Furthermore, it has certain restrictions guaranteeing the user freedom from DRM. If you want to sell a game, you probably don’t want to use content that’s released under this license, as you will be required to distribute the source code, and your users will be allowed to distribute it as well.
GNU LGPL 3
This license requires you to make the source code available for that content only. You may use this content in a non-open-source project, provided that you distribute the source of the content as well as any modifications you make to it under the same license. It does not, however, require you to distribute your entire project under the GNU LGPL. It is generally safe for commercial games to use content released under this license, provided they distribute the content and any modifications to it for free.
GNU GPL 2
This license, like Creative Commons Attribution-Share Alike 3.0 above, requires you to release your entire project under the same license. If you want to sell a game, you probably don’t want to use content that’s released under this license, as you will be required to distribute the source code, and your users will be allowed to distribute it as well.
GNU LGPL 2
This license requires you to make the source code available for that content only. You may use this content in a non-open-source project, provided that you distribute the source of the content as well as any modifications you make to it under the same license. It does not, however, require you to distribute your entire project under the GNU LGPL. It is generally safe for commercial games to use content released under this license, provided they distribute the content and any modifications to it for free.
Copyright-Only Dedication (Public Domain)
This license is equivalent to the Public Domain. There are therefore no legal concerns with using it, and it is safe to use in any project.
Just to reiterate, these notes are based on my understanding of these licenses, and should be taken with a grain of salt. If you notice anything incorrect here, please contact me.