The purpose of having a public domain is to retain culture.
It ensures that you don't have artistic works building dust buried within a corporate regime, no one being to hear or see or read them because the corporation (usually) doesn't see a way to monetize them. Or they feel it's not worth the effort.
Interestingly, in the case of music, there never was a copyright on performances, and a U.S. judge somewhere ruled if that was the case (it was, at least federally), then everything was copyrighted.
The Music Modernization Act attempted to correct that. Performances are protected for 100 years after publication currently until the other, more recent copyright laws, kick in. The musical composition itself is protected for 95 years. No life of the author(s) or any such nonsense.
That means that all music published (as sheet music) 1930 and prior is in the public domain. And the performances of that music that were released 1925 and prior are in the public domain. In the U.S. anyway.
As I've gone through the music of that period (1925 and prior), to determine copyright in Canada (which had, for those years, copyrights of 50 years for performances, and life plus 50 years for the music), I'm seeing more and more online. People are rediscovering this great music and they like it - even in spite of the rather poor sound quality (it all comes from old and used 78's after all, with acoustic recordings) and some of the rather backward cultural references of the time. This public domain thing works!
Regarding some of the casual racism, misogyny, etc., the music is a product of the times. I look at it and see how far we've come (in some countries at least), while enjoying the tunes.
They did have some taste back then. The charts were filled with all types of genres, including dance, ballads, jazz, even opera and classical.
There is even an entire recording of Faust, the Charles Gounod opera. I can't imagine the logistics of crowding an orchestra, and all the singers around a horn in the studio to record the thing. Pretty amazing.
The Public Domain Project is cool resource (but it seems to me to have server issues often) https://en.publicdomainproject.org/index.php/Main_Page "The Public Domain Project is a open-minded (open to all persons) project for the conservation and utilization of Public Domain music and film material.
Another is the Public Domain Sherpa
https://www.publicdomainsherpa.com/public-domain-recordings.html
This list of places where you can find public domain recordings includes resources that offer spoken word recordings and music recordings. Some resources offer both — listed those sites in the Music section.
Great resources if you live in the U.S.
All countries have their own copyright laws, however, so it is limited to that country. It can be a great resource to find material, however, that isn't available anywhere else.
There are all sorts of weird issues when dealing with other countries. For example, generally, once a work enters the public domain it stays there.
However, when the UK was in the EU, it was forced to put public domain material back into copyright when the EU changed its rules to life + 70 for authors. If the work was copyrighted anywhere in the EU when the law went into effect, then the UK had to essentially recopyright it.
The UK also has a broadcast copyright - 50 years. No such copyright exists in either the U.S. or Canada, and as a result, most OTR falls into a gray area.
Then there's the Rule of the Shorter Term, which states that if a work is in the public domain in its country of origin, then it falls into the public domain in any other country following that rule.
Canada theoretically follows the rule, and since radio shows such as The Goon Show is in the public domain in the UK, then it should be in the public domain in Canada. The U.S. doesn't follow the rule, so the status of at least that OTR show is up in the air.
I've done some more research for public domain material.
From what I can gather, there is absolutely no copyright protection for radio broadcasts in the UK prior to June 1, 1957. There were amendments to the rules in 1956 to forbid recording radio broadcasts for commercial use - you could still do it for private use. Who knows if what we do is considered private use.
But if you're in a country that uses the Rule of the Shorter Term, then those former recordings are in the public domain, regardless of its own copyright laws. That includes a whole whack of OTR from the BBC, including various Sherlock Holmes series, The Goon Show and plenty more.
Found this excellent resource...
https://stacker.com/stories/music/50-popular-songs-public-domain
50 popular songs in the public domain
"The year 1925 was significant in American history: Calvin Coolidge became president, the Harlem Renaissance was in full swing, the Scopes Trial began, the Chrysler Corporation was founded, and New York City officially became the largest city in the world. It was also a big year for the arts: ... Originally, these works would have been free to use 75 years after publication, or in 2001. However, in 1998, Congress extended the copyright laws an additional 20 years, keeping these works out of the public domain for that much longer.
In honor of this year's class of works—arguably one of the best to date—entering the public domain, Stacker compiled a list of 50 of the most-listened to songs in the Public Domain, chosen from the Public Domain's list of the 698 most popular songs within. ...."
https://stacker.com/stories/music/50-popular-songs-public-domain
The resource I used to create my playlists was Joel Whitburn's billboard rankings for the years 1929 and prior. While it didn't indicate the public domain status, it was an excellent go to to identify popular songs.