As with images and text your use of recorded sounds is most clearly legally viable when you have created the recording yourself.
Music and other recorded sound is covered by US Copyright Law, and the Fair Use provisions of US Copyright Law, in much the same way that text and images are. Copyright law applies to music and lyrics, as well as in a performance (Title 17 USC, Chapter 11).
Note: Under Creative Commons licenses synching music to images is 'transforming the music', so you cannot legally use a song with a CC "No Derivative Works" license (a CC BY-ND 4.0 or a CC BY-NC-ND 4.0) in a video. You need to use music that is licensed under a CC0, a CC BY 4.0, a CC BY-SA 4.0, or possibly a CC BY-NC-SA 4.0, license. Always make sure to properly credit the musician and the track, as well as express the CC license the track is under.
The following sites offer music, often under a Creative Commons License, for your reuse:
The Internet Archive is an ever-expanding digital library with older versions of websites and other cultural artifacts. Items in these archives are intended to be listened to or viewed; information on other reuse is often available beside the streaming or download link, and additional information is available here. The search options for audio include: