My understanding is that copyright of US sound recordings are governed at the state rather than federal level. Say you wanted to create an App or website for classic early recordings - you would have to get permission from all 50 states plus DC, and other territories in oder to proceed! Good luck with that. However, recordings made in Europe prior to 1961 are in the public domain. In 2011, EU copyright was extended from 50 years to 70 years, but this did not apply to any recording that was already in the public domain.