Fair use of material is a grey area governed by the Digital Millennium Copyright Act (DMCA). You can hire a lawyer to translate it into English. Claiming "Fair use" is really a last ditch effort to not break the law. You might be able to use copyrighted material if it is for non-profit, but that doesn't mean YouTube won't put ads over your video. It might make a difference if you are only using a portion of the music. It might matter that SE is not commercially sold, but having that song out there might affect SE's potential market value, which YouTube doesn't want to be responsible for. Best bet is to use YouTube's own royalty free library of music tracks and add that to your video. Or even better yet, get a license from the makers of the music in SE saying you can use it. A note saying "Yeah, it's ok" in a forum won't cut it. You must contact them directly and pay. One final option is to distort the music so that it is not detected by their tracking system called ContentID, by either changing the speed or adding a filter to it such as a voice synth. I'm sure you knew all this already. This message will self destruct in 10 minutes.
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