I think the best advice I can offer would be that if you are in any doubt, contact the author of the assets in question directly and explain how you wish to use them and whether they give their permissions for that. If they require attribution, then perhaps ask them directly if there is any specific text that they would like mentioned on your Credits scene for example.
Regarding FreeSound.org, I found this under their Frequently Asked Questions;
What do I need to do to legally use the files on freesound?
Well, it depends on what you want to do and which files you want to use. First of all, freesound lets the user select one of three licenses for his sounds. And, we used to have a 4th license, which complicates matters. Depending on the license there are things you can and can’t do with the files. Let’s start with the licenses. Creative commons has a really nice page explaining them:
zero (cc0): http://creativecommons.org/publicdomain/zero/1.0/
attribution (by): http://creativecommons.org/licenses/by/3.0/
attribution noncommercial (by-nc): http://creativecommons.org/licenses/by-nc/3.0/
We aren’t lawyers so this isn’t legal advice, but here’s our summary: for the “zero” license you can do pretty much what you want with the sound. You could even sell the sound, … but you can’t claim you are the author! For “attribution” you should always mention the original creators of the sounds when you use them. “Noncommercial” works like attribution, but you can’t earn any money with the piece of work you create! As with all licenses the original creator can give you permission to use the sound outside of the original license.
That does explain it fairly well I think, but they do go on to mention in the next paragraph that they cannot guarantee that one of their members hasn’t uploaded something which breaks the rules, sharing something for example which they do not have permissions to share, if you were to then use this you would be breaking the authors copyright of the asset in question I believe, albeit unknowingly.
Unknowingly however, may however not be the best defense and again I would suggest that if you are ever uncertain, contacting the author directly would be the best approach. If it were me, I would be delighted to hear from someone wishing to use something I had created and perhaps a small link back to the author’s website etc is a reasonable request for using their assets.
Regarding Unity’s Asset Store, again they have a Terms and Conditions page, here’s a relevant extract;
You may use Unity Asset Store to browse, locate, and download Assets (defined as (i) software or software development kits designed in order to facilitate the development of electronic games and interactive media (ii) content (for example – without limitation – computer graphics, including 3D computer graphics, sounds and music), tutorials and other digital materials created in order to become integrated parts of electronic games). Some of these Assets may be offered by Unity while others may be made available by Providers. You agree that Unity is not responsible for any Asset on the Unity Asset Store that originates from a source other than Unity. Additionally, some Assets may be made available to you at no charge while other Assets may be purchased for a fee. You agree that you are solely responsible for all fees associated with purchases you make on the Unity Asset Store.
Unless you have been specifically permitted to do so in a separate agreement with Unity and except as permitted under the Unity-EULA, you agree that you will not reproduce, duplicate, copy, sell, trade or resell any Asset that you have acquired from the Unity Asset Store for any purpose.
Regarding the above, I believe this is specifically related to the asset itself as opposed to it being in your game. So, you could sell your game using an asset from the Asset Store, but if you were to setup a website and start selling the actual asset itself, that is allowed.
This one is also worth checking and being aware of;
- Unity Services and Third Party Assets and Services
Some components of Assets (whether developed by Unity or third parties) may also be governed by applicable open source software licenses. In the event of a conflict between the Terms, the applicable EULA and any such licenses, the open source software licenses shall prevail with respect to those components.
More of an issue with a script that relies on another library which is provides perhaps as opposed to a 3D character. I wouldn’t necessarily assume that just because a library was bundled with an asset on the Asset Store it is ok to just sell your game which uses it. I would consider searching for the name of the library in question, reading their specific terms and again, if in doubt, contact the author.
And from their Asset Store FAQ;
I hope the above is of some help