In the description of the upload, Brad gave credit to Mine Timelapser for providing the soundfont for K.K Slider’s instrumentals. This has all of K.K. Slider’s notes available to download for composers that wish to recreate songs in the style of the guitar-playing dog.
Source: wiktionary
It quite incredibly recreates the entire 43-minute album with the Super Mario 64 soundfont (a soundfont is essentially all the files used to create a given soundtrack), and I can only speculate at the amount of time that's gone into this.
Source: wiktionary
"About two years ago I made a version of Roygbiv by Boards Of Canada in the Mario 64 soundfont," he said.
Source: wiktionary