Songs receive automatic copyright protection if they were created after 1978, which means that the song's owner doesn't have to register the rights to the song. This can make it more challenging to track down copyright holders, but it also means you should assume that any song you encounter is copyrighted. You may be able to negotiate an agreement to purchase the copyrights to the song, and some song creators offer licenses to people who wish to publicly use their work.
Finding Copyright Holder
To buy the rights to a song, you'll need to find the copyright holder. Check with the U.S. Copyright Office to see if the song is registered; if it is, contact the person to whom the song is registered. If registration records don't exist, contact the source where you found the song. Website administrators, radio stations and disc jockeys can often provide information about who owns the song, and the song's writer or performer may be the copyright holder.
Buying Copyrights
You'll have to negotiate a contract to buy the rights to the song, and your contract should explicitly state that the owner is relinquishing all rights. If the song is a popular one, this could costs hundreds of thousands or even millions of dollars, so be prepared to negotiate a fair price. You won't be able to post the song online, play it on a radio station or alter the song in any way until the copyright holder has signed over her rights.
Work for Hire
If you're working with a new songwriter to create a song, a work for hire agreement can automatically assign the copyrights to you; that means you won't have to buy them later. Draw up a contract indicating that the writer is creating the song solely for your exclusive use and that, when he is paid and you receive the song, he forfeits all copyrights to the song.
Licensing
Buying the copyrights to a popular song can be cost prohibitive, but you can get access to the song by purchasing a license instead. Many music companies offer licenses to DJs, radio stations, performers and other people who want to publish or otherwise use the song. Contact the owner of the song's rights to ask about a license agreement. You'll usually pay a recurring fee to use the song in an unlimited way. If you want to use it just once, you can pay a one-time-use fee.