These assets would include images, sounds, and anything else in the project besides the source code. I own the copyright to these assets.
To be clear, I'm asking how I would actually apply the license in my repository in practice.
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They are your files (well, you hold the copyrights) so you can convey them under whatever licence you like. If you decide that CC-BY-SA 4 would be more appropriate than GPLv3, then simply stop conveying them under GPLv3. Remove any text that suggests these files are conveyed under GPLv3, and instead attach text making it clear they're conveyed under CC-BY-SA 4 instead.
What you cannot do is somehow call back copies that were already distributed under GPLv3. People who have those copies will continue to use them under the terms under which they were conveyed, ie, GPLv3, which requires that if they convey copies onwards those copies will also be under GPLv3.