YouTube Annouces the Audio Library. This means you can download the music and use on your videos uploaded to YouTube. To access the Audio Library sigin into your YouTube channel. Click on Video Manager. The screen shot below shows "how" to find the Library.
On Left side after you click Video Manager then Click on Creation Tools and then Audio Library.
You can play songs before you downlaod. Make sure to read Terms and Conditions. The down arrow far left is for you to download.
Very sure Google would like to see more folks use permission based music.