Copyrights give the authors of artistic works the right to keep other people from using and copying their works without the authors' permission. Under U.S. copyright law, a slideshow is considered to be an "audiovisual work" or "multimedia work" that must comply with American copyright rules. If you add music to a slideshow, that music is governed by the same copyright laws that protect music appearing in sheet music and on sound recordings.
If you are adding original music that you personally have composed to a slideshow, you own the copyright on that music. If you intend showing the slideshow publicly, it is possible, although not necessary, to formally register the music with the U.S. Copyright Office. Your slideshow software may also contain free music clips that are original compositions specifically composed or purchased for use with the software program.
Copyrights to musical scores eventually expire, and the sheet music enters what is called the "public domain," meaning that anyone may use the sheet music without the permission of the original author or his heirs. Sheet music published in 1922 or earlier has fallen into the public domain, and you may use the music for your slideshow, provided that the copyright was never renewed. Check the sheet music of the composition and make sure that it does not contain information showing that the copyright was renewed after 1922. If you are still uncertain, contact the U.S. Copyright Office and ask if the copyright was renewed after 1922.
Copyright laws governing sound recordings of performances, such as CDs, record albums and tapes, changed repeatedly in the 20th century, leaving some recorded performances of music in the public domain and some still copyrighted, depending on the date of publication and whether the legal formalities of copyright registration were carefully followed. If you have a sound recording created by someone other than yourself that you would like to include in your slideshow, review the Cornell University Copyright Information Center's brochure, "Copyright Term and the Public Domain in the United States." Under "Sound Recordings Published in the United States," match the date when the recording was published against the dates in the brochure to see if your sound recording is still copyrighted.
If the music you would like to use in your slideshow is still protected by the original composer's copyright, you have several alternatives. Many composers are legally represented by one of two organizations: the American Society of Composers, Authors and Publishers; or Broadcast Music, Inc. You can contact the organization representing the original composer and pay for the right to use the music. If you cannot work out a deal with the original composer's representatives, you can get in touch with organizations providing huge online music libraries, which sell the right to use similar music by other composers.