Great points mdwh, all of them. In addition to confusion around GPL, I know there has also been a lot of confusion here regarding CC-BY-SA in closed source games. I did a little digging (all emphasis below is mine). Keep in mind that I am definitely not a lawyer.

First, see here under Potential Compatible Licenses (I know it's listed as for 4.0 versions, but it's not a legal document, rather it just expresses how CC thinks about things).

[While] CC is suitable for content and not software, the reality is that increasingly, people are creating works that mix content and code in ways that create uncertainty around how the licenses interact.

So they at least acknowledge possible ambiguities.

Another is interactive games and art, which generate new artistic works by using code to combine existing smaller works. Video games are code, but also art, and some blur the distinction so much that you can’t separate out the elements to treat them as a simple aggregation. Many generate new, original graphics or music based on the underlying code and the player’s actions; where they are built from BY or BY-SA licensed pieces, it may be necessary to ensure that they are compatible.

I read this as "if your code uses CC content to produce new, original graphics or sounds, then your code may need to be licensed compatibally with CC." However, a video game does not by definition appear to trigger this. My (definitely non-lawyer) interpretation is that code moving graphics around, or cycling through animation frames, does not constitute creating new, original art, and therefore does not need to be compatible.

Now, looking at the actual CC-BY-SA 3.0 license:

"Collective Work" means a work, such as a periodical issue, anthology or encyclopedia, in which the Work in its entirety in unmodified form, along with one or more other contributions, constituting separate and independent works in themselves, are assembled into a collective whole. A work that constitutes a Collective Work will not be considered a Derivative Work (as defined below) for the purposes of this License.

Further down, looking at "Derivative Work" I think the most telling lines are

such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which the Work may be recast, transformed, or adapted

In my opinion, none of the examples they list here seem similar to "inclusion in a video game" (except perhaps "motion picture version"???), whereas I think that a "Collective Work" is a better maatch for video games, at least insofar as the game code is not manipulating the CC-BY-SA content to produce novel art (I doubt that moving a sprite, or animating a sprite counts as sufficiently novel), as discussed above. If however, the sprites were provided as a spritesheet and you split them up to display individually, I believe that the inidividually split sprites are a derivative work, and you would be requried to release them under a compatible license.

For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered a Derivative Work

So interestingly, I would say that if you download a sword-slash sound effect licensed under CC-BY-SA, and create your own custom sword-slashing animted sprite, and your game synchronizes the playing of the sound over the animation, then I would interpret this to mean that the "slash SFX / slash sprite" combination is a derivative of the SFX, and must be released under a compatible license.

tl;dr - Looking at how CC thinks about compatibility, and the text of the license, I (who am definitely not a lawyer) would interpret that under most cases, it is safe to use CC-BY-SA 3.0 content in closed source games, as CC seems to think of code (including video games) and content separately. Instead, I think that video games may more closely resemble "Collective Works". You may run into trouble if your gameplay mechanic focusses on manipulating artwork to create original art. You may also run into trouble if you synchronize sound effects or music to non-freely licensed art.

Again though... I'm not a lawyer and this is not legal advice. In fact, I think that no matter what goes into the FAQ, it deserves to have a warning saying that it is not legal advice, and it is the responsibility of the user to make sure that they are using the artwork in a way that is compatible with the appropriate license.