Music can enhance the audio version of your book. But when creating an audiobook, you need to be careful to respect the intellectual property rights of the artists who created the music. There is music in the public domain (music that is not covered by copyright) but determining the public domain status of a work is not necessarily simple.
I have had versions of the conversation that follows with many authors and publishers over the years. What you see below addresses many of the issues you need to consider when trying to determine what music may be available in the public domain. (Note: I am not an attorney and this should not be considered legal advice. I am just someone who spends a lot of time dealing with copyright issues and researching the applicable laws. I may not have all of the answers, but I at least know most of the questions.)
I am turning my book into an audio book, and I want to add music to it. I have an English-language recording of a traditional German folk song from the 1800s that fits my book perfectly. Because the song is so old, there would not be any copyright issues with using it, would there?
There are several issues that must be considered before you use the song. First of all, copyright laws in other countries are not necessarily the same as in the United States. This song is old enough that it is probably in the public domain both in the U.S. and Germany.
However, the recording you have probably is not. The performers of the song would hold a copyright on the recording of the performance. Unless the copyright has expired or otherwise does not exist (e.g., the copyright holder chose to place the work in the public domain), you would need permission from the artists to use their work.
It would be more work, but I guess I could record the song myself. I have sheet music my band can use.
Not so fast, Sparky. The arrangement in that sheet music may be covered by copyright. Performing and recording the song, as written in the sheet music, may require permission from the arranger.
<Sigh> OK, so my band and I will come up with our own arrangement. Do you see any problem with that?
At least one. You mentioned that you are planning to use an English-language version of a traditional German song. Is the translation of the lyrics copyrighted? Even if the original work is not covered by copyright, the translator may hold a copyright on his or her translation.
These points may seem like nitpicking, but they matter to the copyright holder. As someone who creates intellectual property, you would not want someone using your work without your permission. Extending the same courtesy to others is not just the friendly thing to do, it is the law.
There are lots of copyright, public domain and other intellectual property law issues that require consulting with an attorney to understand how the law applies. However, having a basic understanding of copyright law can avert many problems and help you figure out what questions you need to ask and when you need to get an attorney involved. If you want to know more about determining the public domain status of a work, my favorite reference is The Public Domain: How to Find & Use Copyright-Free Writings, Music, Art & More by Stephen Frishman. Get your copy now at Amazon.com.