YouTube Audio Library. The soundtracks in YouTube Audio Library can be used for commercial or non-commercial purposes. Their use isn't limited to YouTube - you can use them in presentations or anywhere else. https://www.youtube.com/channel/UCht8qITGkBvXKsR1Byln-wA
The Free Music Archive allows any musician or artist to make their tracks freely available. The collection is curated to filter out people who may breach copyright. uploading http://freemusicarchive.org/
Beware of using music that claims to be "Creative Commons" or "Free to use" as it is difficult to tell if the copyright really does belong to the person making that claim. They would need to have rights to the score, the performance and the recording.