As with images and text your use of recorded sounds is most clearly legally viable when you have created the recording yourself. At Journalists' Toolkit Mindy McAdams offers a very useful list of resources about audio production and journalistic interviewing. If your use of audio includes interviews, narration, voice-overs, ambient noise, or natural sounds - whether or not you recorded the sound yourself - you should take a look at her links.
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 provides for a copyright in the music and lyrics, and for a copyright in a performance (Title 17 USC, Chapter 11). For example, Wolfgang Amadeus Mozart wrote “Eine kleine Nachtmusik” in 1787, long enough ago that it is obviously in the public domain. This recording of it was originally made on December 17, 1936 by the Vienna Philharmonic Orchestra, and their copyright on that recording has apparently lapsed, enabling it to be digitally copied to the Internet Archive. But copyright law gives a digital version its own copyright from the moment it is created (i.e. "fixed in an immutable form"), necessitating the use of a Creative Commons 'Attribution-NonCommercial-ShareAlike' License for legally-uncomplicated distribution of a clearly Public Domain work.
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:
Audiofarm is in business to help people broadcast their audio files to as many other people as possible. Many owners of audio content posted at Audiofarm have released their audio content under Creative Commons licenses. Available genres include: News & Radio, Dance, Soundtracks, Electronic, Audiobooks, Podcasts, Interviews, Comedy, World Music, Hip Hop, Rock Music, Indian Classic, Religious, Documentary, Voice Acting, and Game Audio. Audiofarm provides API access.
BeatPick.com is a "one-stop music licensing shop". If you hold the copyright for some music you can use Beatpick to license your music for commercial purposes. If you are unable to find an appropriate public domain or Creative Commons piece of music elsewhere you can license music from Beatpick. Music licensing at BeatPick. can vary from 30 Euro to 30,000 Euro, depending on the type of project you are working upon. You can search by genre, mood, vocal/instrumental, language, keywords, sounds like, song topics, artist or title, main instrument, and tempo. For a comprehensive price list click “License Song” next to any song, answer the questions and get an instant quote.
ccMixter is a community music remixing site featuring remixes and samples under Creative Commons licenses. You are free to download and sample from music on this site and share the results with anyone, anywhere, anytime. Some songs might have certain restrictions, depending on their specific licenses. Each submission is marked clearly with the license that applies to it. ccMixter offers several ways to search its contents, explore the tabs on the main page, their music discover tool "dig.ccMixter", and scroll/click through "Everything You Can Do and See at ccMixter". Clear instruction on proper attribution are provided.
Free Music Archive is an interactive library of legal audio downloads curated by radio station WFMU. Every MP3 in The Free Music Archive is pre-cleared for specific uses by the rights-holder (FAQ). The Search Options include Mood, Artist/Album/Song Title, Genre, Instrumental Only, Duration, Tempo/BPM, and License Type. To see the license for a specific song click on the track title, then on the song page look at the right hand column to see the license for that song.
Freesound is a collaborative database with more than 200,000 Creative Commons Licensed sounds that you may browse, download, and share. The Search Options include Tags, File names, Descriptions, Pack names, Sound Ids, User names, Duration, Geotagged Sounds, Type, Sample Rate, Bitdepth, and Bitrate.
FreeSoundtrackMusic.com offers both 'free' soundtrack music and 'free for a fee' soundtrack music. Attribution is required for 'free' use. Registered Users (cost $3.50) receive access to advanced search functions.
Opsound is "a gift economy in action". Musicians and sound artists are invited to add their work to the Opsound pool All sounds available through Opsound are either under the Creative Commons Attribution-ShareAlike license or have been placed in the public domain. You can search by New, Tags, Genres, and Artist. (If you find a dead link try using the Wayback Machine.)
SoundBible.com offers thousands of Creative Commons Licensed or Public Domain free sound effects and royalty-free sounds that can be used in a variety of sound projects including Video Sound Scores, Movie Scoring, Game Design, Powerpoint Presentations, Prank Calls, Sound Boards, and more. They also offer links to other resources and if you like their sounds a link back to the SoundBible site is appreciated.
SoundClick offers 635,069 songs under one or another Creative Commons licenses. [Change the license type to "Creative Commons (free)" and then select your genre. The License/Genre Search returns only 1,000 results at a time and most of their genres include more songs than that, you may wish to also try their google-powered search with its drop-down options.]
SoundCloud is a "social sound platform" where anyone can create a free account and share the sounds they create. Much of the music and other sounds on SoundCloud is available under a Creative Commons License. On this search page, under 'fliter results' on the left side, select the type of license you would like to use, and then enter your search terms.