OK, at the time of this writing, most, if not all, of the music in the Youtube Audio Library are under the "YouTube Audio Library License" which has this specific text when you click on it for more information:
-
You can use this audio track in any of your videos, including videos that you monetize. No attribution is required.
-
YouTube may credit the artist and link the Audio Library from your video.
-
You may not make available, distribute or perform the music files from this library separately from videos and other content into which you have incorporated these music files (e.g., standalone distribution of these files is not permitted).