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is it okay to use music/background art from a copyrighted commercial game in an unrelated, non-commercial game?
That's much
more of a no-no really.
Using in-game content to create your own art that comments on the original is a sort of derivative work... as mentioned, fanvideos/machinima are often allowed to an extent. Telling a warcraft story using your own unique and carefully posed screenshots from within the game is legally questionable unless you have permission, but it's an area of copyright that is currently evolving.
Ripping content directly from a game and using it as-is in a different game, on the other hand, is pretty much outright theft. There's no gray area there, no possibility of fanwork permission, because if it's unrelated it's not a fanwork.
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I realize that sound effects are similarly copyrighted assets, but I wonder, how likely is it that someone would notice? Especially if using original art/ story/ music.
There are sound geeks out there who will notice. I have seen lawsuits filed over people realising that the laser sounds in game X were actually ripped from game Y.
Now, obviously, if only 200 people ever play your little free game it's unlikely that one of those 200 people will be a sound geek. But still.
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Because my fantasy adventure needs battle noises and where am I supposed to get the grunts of an angry ogre? Can't really hire a sound designer either, now can I?
a) There are sound effect packages of fantasy sounds you can buy, fairly cheap.
b) There are individual sound effects you can buy, fairly cheap.
c) Get a microphone and grunt.

I do all of the above - I
like making my own sounds when it's feasible, but some things pros are much better at than I am.