Is it OK to mix assets that are Creative Commons 3.0 BY-SA with other assets that are in the Public Domain in the same project? Thanks.
For instance here.
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If you abide by the terms of all of the licenses, sure. Combining a BY-SA work with a public domain work results in a BY-SA work.
What "combining" means might include using them in the a project. It's legally fuzzy whether a whole media like a game is a "derived work" of its individual assets or not and may well depend on how critical a judge or jury deems that asset is to the game.
Safest bet is always to just make everything in-house from scratch.
Public Domain means either that the copyright has expired and thus it may be used freely or it means that the author relinquished all rights they have to the work as far as legally possible (in some jurisdictions it is impossible to do that). That also means that a PD work can be taken and relicensed under a different license like CC-BY-SA, GPL, BSD, MIT or a proprietary work. PD is not viral.
When your alterations to a PD work have enough merit to be eligible for copyright themself, these altered versions lose their PD status. Example: Beethoven's Symphony Number 5 is public domain (author died more than 70 years ago) but an audio recording of an orchestra playing it is not public domain, because their interpretation is eligible for copyright. A sheet displaying the notes can also be copyrighted, because the layouting and design is eligible for copyright, even though the content is not.
It would, however, be fair for end-users and licensees of your work when you would state which parts are unaltered from public domain sources so they know that they can reuse them individually without having to adhere to CC-BY-SA.