When you swap audio on YouTube with a recommended music track, you have to opportunity to make either the original audio track louder, or the swapped music track louder, or make it equal.
Here's a video where I chose equal.
I trimmed the video in QuickTime Player 10.1 first, then sent directly to YouTube from within the QuickTime Player app.
One thing to notice is that the clip I selected is only 30 seconds long so, after it plays, the video's original audio plays.
Next time I'll try it with a longer audio.