Fair use is an important part of copyright in the United States, but it should not be relied on. It is always best to secure permission, pay for a license, or use openly licensed materials before relying on a fair use defense.
Transformative use is another aspect to consider: Is your use of copyrighted material part of a commentary, criticism, reporting, or creative reworking? This might provide another avenue for fair use, but the principle of "transformative use" is still developing.
Music copyright is notoriously confusing. The copyright of a written piece of music, such as a score, has a separate copyright from a recording. Both are covered by copyright because they are "fixed" in a medium. Thus, a piece of music can be in the public domain, but a recording of that piece is separately copyrighted.
Radio broadcasts have their own rules and regulations for the performance of music. Most music requires a license to be broadcast even short snippets (unless used in a transformative way). Read Copyright Basics for Radio Stations and Webcasters for more information.
Music can also be licensed under Creative Commons licenses, and searching for samples that can be reused and remixed freely can be done through the databases listed here.
Photograph copyrights are owned by the photographer, or the venue who the photographer sells the rights to.
If looking for images that can be reused and modified, try using the Creative Commons Search.