Who would come after you if you decided to use music from the 1930's, say, in a podcast, without permission? Who would you get permission from?
As the law's currently written works copyrighted after 1922 are protected for 95 years (and ones after 1978 even longer). So everything between registered between 1922 and 1978 is off-limits until 2019 at the earliest.
If for example, you wanted to use a recording from 1923 in your podcast, you would have to get permission from the current copyright owner. For an RCA Victor release, for example, you would have to contact Sony/BMG. Okeh Records was independent in the 1920s but was eventually purchased by Columbia, which was bought by Sony and is now part of Sony/BMG -- who you would have to contact to get permission.
But what if it's an "orphan" work? That is, what about works copyrighted by companies that no longer exist, and whose assets were never purchased by another company? Under the current system, these abandoned works do not automatically lapse into public domain. It is incumbent upon the user to make a diligent search for the copyright owners, and even if they cannot be located, the work is still considered to be used without proper clearance.
Legislation has been introduced to force the issue and get orphan works to lapse into public domain -- but it's being fought by the RIAA, MPAA, and other content owners. Why? Because they don't want any of their material that -- through oversight -- may be considered abandoned to automatically become public domain.
And the effect?
Books by publishers long gone molder on library shelves. Archivists don't dare scan them to preserve their content because they're not in the public domain.
Movies from film companies that collapsed in the Great Depression disintegrate in their cans. Preservationists can't reproduce them because they're not in the public domain.
Recordings from defunct labels remain unheard. Sound engineers can't transfer them and present them to the public because they're not in the public domain.
And so a large part of our past disappears into the mist of time while the Big Media dog furiously barks to keep the public at bay. The record labels, publishers and movie studios can't make any money from this material. It isn't useful to them -- but by God no one else is going to use it either!