Thanks for reply.

I understand what you are saying, but when i thought that there are online pages or programs to download videos from YouTube, I believed that there could be a possibility of doing what I want legally. But leaving that topic aside, let me explain my issue:

If I download a song, add notes and generate a file with those notes, I can upload this file on my website for example but what i can't do is upload the song file due to copyright issues so the user needs to find the song in internet and download that song somehow, but they could have downloaded an audio with a different delay from the audio that I mapped, then the notes will not match, that's the main issue that i have. 

Maybe I can provide the link of the youtube video that I mapped specifically and that when you click on a button in my game it will copy to clipboard the url of the youtube video that you have to download somehow. So once you have the audio downloaded you just put it in my game and you can play without any delay or offset issues.

Another thing I can do without having to link anything from youtube (although more complicated and less comfortable for the user) is that the users downloads the somehow and they can configure that delay with some kind of tool in the game in case the audios have a different delay, then save that offset and play.

The idea of all this is to create a portfolio, play with friends and share it, I don't intend to sell anything, but I still want to make it 100% legal.