Is downloading music from youtube, then using it on scratcth ilegal? I don't know myself, but if it is, lots of people have done it!
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In my projects, almost all of the music in them are downloads from youtube, becasue, well, the scracth music libary doesn't have ANY of the sound effects I neeed, and all the music in it isn't usefull to me. I am worried about this, and I'm only ten!

It's a violation of youtube's policies and potentially a violation of copyright law, so I wouldn't recommend doing it.

However, most people get away with it since basically nobody targets Scratch as nobody is making any money off their projects, and it's a nonprofit.