In the U.S., works published before 1978 without a proper copyright notice are public domain if the publication was a "general" publication and was published with permission. See comprehensive chart:
http://www.copyright.cornell.edu/training/Hirtle_Public_Domain.htm

However, foreign works were given special treatment and most were exempted from this requirement. The copyright in such works was retroactively restored. See:
http://www.copyright.gov/gatt.html

(The restoration may also apply to sound recordings fixed before February 15, 1972.)

One law firm is challenging this. They are asking for your experiences with this law, if any:
http://cyberlaw.stanford.edu/about/cases/golan_v_ashcroft.shtml

Looks interesting

Lawrence Golan, a music director, explained how classical music works were effected by the changes in copyright laws: "These pieces used to be in the public domain and were available for a reasonable purchase price (between $50 to $200 each for the complete set of orchestral parts). However, after the grant of restored copyrights, they are no longer available for purchase. Even with discounts offered by the publishers to educational institutions, the sheet music now costs anywhere from $300 to $1,000 simply to rent for a single performance. After the performance, the rented sheet music must be returned. Because my budget for purchasing and/or renting music is not unlimited, I cannot afford to rent many of these works any longer. The removal of these works from the public domain has severely impaired my ability to teach my students. Their education and the education of the listening public have been forever harmed because of this law."
Rent sheet music.
This means that schools perform the same pieces year after year - the same classical works, the same musicals.
In Pennsylvania, schools are funded by real estate property taxes and less and less by the state and federal government. What do school boards cut in order to trim the budget? Music.
It seems so clear-cut. The Sonny Bono Copyright Extension Act is a disaster to education. Why has the government (No Child Left Behind) been fighting for almost three years for this legislative error?

What classical works? All published classical period works are public domain as is. Or did you simply mean "classical" as a style of music?

I guess I should also add that the law that says works published prior to 1978 without copyright notice are public domain, does NOT apply to music unless the work was also published as printed material. (e.g. notation, tab, etc.) Because of Orrin Hatch's Senate (someone I used to respect) and the case cited below, publishing an LP before 1978 was not a publication of the song on it. Go figure!!

http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=9th&navby=case&no=9856145

Re: classical.

Ah, OK I found where you quoted from.

http://notabug.com/golan/1