What is the best software (paid and free) to use for remix videos on youtube. Here is what I want to do:

I want to put my remix audio behind the original video of the song I remixed in place of the original audio.

To do this, would I need the original music video file or would I be able to rip the video from Youtube and just remove the audio? Whats the best way to remove the audio and replace it with mine?

Thanks