Whenever you Go to YouTube and you can see a drop down menu at the top right that says 'Account'. Click the drop down and you'll see a list of your videos.
Under your each particular video, you'll see a button called Audio Swap,Click that button
Now You will come to a screen with several selections of music choices. Select any one that interests you, and click 'Preview' under Step 2 on the right side of the screen. Do this for each of your selected song choices.
When you find a song that you are happy with, click publish. Your current song will be swapped with your new selection.
As simple as that.Your videos audio has been swapped by a song of your selection.