A few years ago, I downloaded this fan-game called Super Mario Bros. X. It automatically came with a folder containing all the music they used, so I decided to post those files on Scratch for people to use in their projects. This way, you don't have to download an entire game and import the sound files into the project yourself! Just see inside, put 'em in your backpack, and use them! It even includes non-Mario songs from other Nintendo games.
Of course, credit to Nintendo for the songs, and the Super Mario Bros. X people for putting them in their downloadable game. No need to give credit to me if you use them.
Fun fact: This project originally went way over the Scratch filesize limit. Turns out sound files are huge. I had to delete and shorten a whole bunch to share this.