Legal Question in Intellectual Property in New York

I would be very wary about taking the path you describe. If it is a "loose" remix as you say, it is nothing more than a derivative work. That being said, making an unauthorized derivative work of a copyrighted song (one that is popular no less) can expose your client and you to copyright liability. This could be very embarrassing if not financially injurious. If at all possible, you should use music that is properly licensed or in the public domain.

I agree. Do not use the loose remix of the popular song in the audio. It is irrelevant that you will be using the video for YOUTUBE. The only one who could give you permission to remix the song or use the song itself is the owner of the copyright to the song. You will need to get permission from them.Or, even easier, if the owner of the song is a member of ASCAP and BMI, you might be able to secure a license for the use of the song by going to http://www.ascap.com/licensing/about.html or www.bmi.com. GOOD LUCK!