The U.S. Copyright Act of 1976 creates a monopoly for authors of original musical works. This means that only the copyright holder is allowed to profit from her creation. Included in this monopoly is the right to perform the work in public. As long as you hold the copyright, you can sing your songs as often as you like, in front of as many people as you want. On the other hand, if someone else performs your work in public without permission or paying a royalty, he may have infringed your copyright.
If you want to perform published music in public, it must be your own original work, or you must have permission from the copyright holder. When it comes to your own piece, originality is a relatively low threshold under U.S. law. Originality is broadly defined and requires only a modicum of creativity. On the other hand, if you borrow parts of other people's work -- through sampling, for example -- you may need to get permission, even if it's just a few seconds of the song.
Another main requirement for copyright protection is that the work must be "fixed" in a tangible medium that others can perceive and refer back to. Tangible media include photographs, videos and written works -- even those saved in a digital format. Once an original work is fixed in a tangible medium, no further action is necessary to invoke the protection of copyright laws. However, if you take the extra step of registering your song or instrumental pieces with the U.S. Copyright Office, it is easier to prove the existence of your copyright. For musicians, this means that if you knock out a clever riff on a guitar, but you don't record it somehow -- video, audio, written on paper -- it is not protected. The talented bystander who overheard you could run out and play your song in the nearest park, and you will have no recourse.
If you choose to publicly perform a song written by someone else -- Paul McCartney, for example -- you don't need to call him and ask permission. Neither does he need to email you if he wants to play your piece. Instead, Sir Paul can report his performance and submit a royalty payment to the appropriate performing rights society, such as American Society of Composers, Authors and Publishers. This is part of the music industry self-governance known as pay-for-play rules. The rules are detailed and intricate; even a large restaurant playing a radio that customers can hear may be required to pay fees. If you or your business are considering playing music that you didn't write, check with an attorney before doing so. You could save yourself a lot of legal headache.
The duration of copyright depends on when the song was first published or fixed in a tangible medium, and copyright laws are complex. Current law dates to 1978, when the Copyright Act of 1976 took effect. Protected works dating back further than 1978 have varying copyright duration times. Any work published or registered before 1923 is currently in the public domain, which means it no longer has copyright protection, and you can play it as loud as you like. The same is true for pieces of music that received copyright protection between 1923 and 1963, but for which the copyright was never renewed. On the other hand, works that were created and renewed during the same time period -- 1923 to 1963 -- are protected by copyright laws for 95 years from the date the work was first published or registered with the Copyright Office. For works that were created before 1978, but never published or registered, the owner receives a single copyright term of 120 years from the date the work was first created. If such works were created before 1978 and published or registered between 1978 and 2002, the copyright will expire Jan. 1, 2048. Finally, any song written after 1978 is protected for the duration of the author's life and for an additional 70 years after her death.