Below are a list of commonly used audio/music websites that can be used for free or near to free. Please follow the site rules on each site for attribution as each has its own guidelines for re-using content on their sites.
Creative Common licensed audio by genres and various artists. Creative Commons, must attribute artist.
Creative Commons music
Free Music Archive
A site filled with downloadable sound Effects. Not free but very inexpensive if you need a particular sound effect for your presentations, videos and class materials.
YouTube Audio Library
YouTube has a selection of free music that can be used. However, you will require to sign up for a YouTube account before accessing this library.