Music and other published works that are in the Public Domain are free to use; since, they are not under copyright protection and therefore do not require any permissions from the author (or former copyright owner) to be used. Works that are in the public domain are free for the taking, sampling, using, copying, reproducing, recording and distributing.
Typically, works in the Public Domain are very old works. For example, ancient published texts like the “Bhagavad Gita” (a pre-Christian, Sanskrit text) are in the public domain…. as well as “newer-old-works” like the Shaker song “Simple Gifts” (music and lyrics written in the United States in the mid 1800’s). The original text, music and lyrics of the works are in the public domain. However, newer translations or compositions based on the original works… as well as sound recordings and arrangement elements of these newer versions are likely covered by copyright protection…. and NOT in the public domain.
What works are in the public domain? Any work or musical score that was published in the United States before 1923 is in the public domain, due to expiration of copyright. Newer works can also be dedicated to the public domain and if a work failed to meet the requirements for copyright protection it will also be in the public domain. Generally, a case by case analysis should to be done on works published in 1923 or later to determine if they are in the public domain because, it’s not always obvious. [Would you have guessed that the Happy Birthday Song is still covered by copyright protection… and not in the public domain?]
Keep in mind that copyright duration and the timeline for works entering the public domain vary country by country.