Here they are in no particular order:
Part of the internet Archive, this site contains thousands of community-contributed Creative Commons licensed audio recordings. Item are free to use, per each specified Creative Commons license.
SoundCloud is the world’s leading social sound platform where anyone can create sounds and share them everywhere. SoundClouds’ dedicated Creative Commons page allows you to find tracks that you can use for free according to the terms provided in the license.
Musopen offers a wealth of free classical music tracks. Founded in 2012, Musopen is a 501(c) 3 non-profit based out of San Francisco, CA. It provides free music recordings, sheet music, apps, and educational materials. You may use five downloads per day for free.
As a librarian, I would be remiss if I did not mention the treasure which the Library of Congress offers free of charge. The recordings are a little scratchy, but they are wonderful and in many cases, free to use.
Avoiding copyright infringement:
Please keep in mind that in all cases, as creators, we need to give credit where it is due. It is critical to attribute the source of music. I usually have a slide at the end of my presentations with the credits. I’m attaching a sample here, which I used in a video. The song “African Bliss” by John Bartmann was a perfect choice for the video.