Are you looking for it on your computer or on the sound list on scratch?
If its on your computer you will just have to find where you saved it to, but scratch comes with a few pre-made ones in its sound folder.
Once you have imported it to the sound section its really simple to add it to a project.
Just drag out the play sound block then click on the little down arrow in the box and look for the sound you want.
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Oh and by the way when you click the button that says your name in the scratch browse it actually takes you to your documents for some reason, that might be confusing you.
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If you want to find some free music that fits your project, try looking here.
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