> I find a cool remix on Youtube, but it doesn't exist anywhere else.

Arggghhh, I'm in the same camp as you! I use a variety of music listening services (Spotify, YouTube and HypeM mainly) and I'd love to have one single source to do this all and share with friends. Two major themes pop up here:

1. Any company that could achieve this would effectively create a monopoly status - one which oddly I feel like I would support.

2. Meta data (tags, etc) for audio files is a mess. A side effect of this is the people trying to "game" the system (i.e. "Vocal Superstars presents a song played in the style of Rihanna's song") or reusing song names (i.e. any song that contains the word Love).