In an effort to keep international relations positive, President Bill Clinton signed a law in 1995 that restored copyright to any international works that fell into the U.S. public domain for failure to comply with U.S. formalities (like copyright notice, registration, and renewal). This means that if a Shostakovich symphony was published after 1923 and failed to renew the copyright with the U.S. Copyright Office, it is still protected, even though the same situation with an American work, like a Gershwin symphony, would place the work in the public domain. This copyright restoration is governed by §104A of the Copyright Act. Click on the circular linked below for more information on how to determine the copyright status for foreign works.
USCO Circular 38b - Copyright Restoration for International Works