The Library of Congress used to have a goal of including complete hard copies, at least for items of US origin and 'good grade' (that is, they aimed to have copies of things such as hardback books that were intended to last, more than, say, ephemera such as the pulp magazines). However, that goal has become an obvious impossibility due to sheer volume. After about 1960, the library began being more selective.
And the situation is infinitely worse for other medias. Not only aren't people trying to preserve them, in many case they have been actively destroyed, in particular television broadcasts.
Two examples of the casualties:
- http://en.wikipedia.org/wiki/List_of_surviving_DuMont_Television_Network_broadcasts lists what survived from a decade of broadcasting on the DuMont network. Everything else was destroyed for various reasons.
- http://en.wikipedia.org/wiki/List_of_The_Avengers_episodes labels most of the first season of the famous TV show as "missing", because the tapes were re-used in the 60s or 70s to save money.
The relevant wikipedia category is http://en.wikipedia.org/wiki/Category:Lost_television_programs. It's hard to believe so much television history has been lost forever.