Let's begin this process with an overview of policies pertaining to the designation of music as public domain. According to the Public Domain Information Information Process, any music and/or lyrics published prior to 1922 is considered public domain within the USA. The FAQ section includes information about the use of public domain music:
Do I have to work from a copy of a work with a 1922 or earlier copyright date?
Any work in the public domain can be freely used by any one in any manner they choose. There is no law which requires you to have any proof of public domain in order to use a public domain work. The problem is that it is virtually impossible to securely determine that a work is in the public domain in the USA unless you have a copy of the work with a copyright date of 1922 or earlier. Most recognizable public domain songs have hundreds of arrangements of the song which are still under copyright protection, and in many instances the arranger has significantly changed the melody or lyrics from public domain versions. So to be confident you are truly using a public domain work, you need to find a public domain version of the work before you begin your project. A reprint from a source you trust or a photocopy from a library are most adequate. You then work exclusively from your public domain source copy to change and arrange the song as you like to create the music you need for your project.
The future is bright... if we focus on music from pre-1922, we can scan, post, and most likely share any of the pieces we uncover. The FAQ section goes on to discuss the legalities of posting scanned Public Domain (PD) music on websites--an activity that is completely legal if the music is confirmed as PD. Once we post the music to our website, it becomes our property under copyright law... that's how sites like this PD-Info one referenced above get away with charging $3 for a copy of a PD piece of music.
