I suspect codecs missing. Open up
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and use the "Download Standard" link at the top. This is a huge codec pack which has a pretty good reputation and which I've been recommending to some others here, too

After installing them, try to run the game again.
Also, as a side note, since you're running Win 8.1, you should consider an upgrade. Windows 10 is still supported (at least until October of this year) and the upgrade should still be possible for free with your genuine 8.1 key.
Might be possible that this would've also solved your problem.