Using YouTube's AudioSwap feature is an easy way to give your silent YouTube video a soundtrack. It's easy, legal, and free. This method removes your existing soundtrack, so only use this on videos that don't have any dialog.
First, log into your YouTube account and add the AudioSwap feature by clicking on the Try AudioSwap button.
Next, go to your My Videos page. Find the video you want to use and press the Replace Audio button.
Next, browse for the music you'd like to use. First select a style of music on the left. Next, select an artist, and then select a song. It tells you how long the song is, so you don't accidentally pick a song that's much shorter than your video.
You can preview the soundtrack along with your video on the right.
Once you've selected your soundtrack, press the Publish Video button. Keep in mind that any existing audio will be deleted from your video.
That's it. You now have a legal soundtrack for your video.