I wouldn't count on the 1923 rule too heavily. I did a little historical fiction and compilation work a few years back, and there are problems with things like Mark Twain stuff even dating back before '23. Sometimes grasping heirs and/or firms which bought copyrights refuse to let go, and they renewed copyrights, running them into the newer stretches designated by later laws, and they're STILL not public domain. There are lots of variables, including the fact that the rights go from the author's death, not the date of publication.

Not an attorney, and clearly the prior writers have set forth the important stuff to know, but I'd hate to see somebody assume there is a blanket 1923 rule, slap stuff on an album or in a book, and get into some lawsuit that could have been headed off.