Take, for example, this episode from Tom & Jerry.
Every movement/expression/feeling the characters display is included on the background music, which is a different thing than the usual sound effects used for a given action; and what impresses me most is the fact that these soundtracks had to be created and recorded episode by episode.
It's also nice to hear that they use the same composition throughout the episode in many different ways. In the case above, the composition used is Chopin's Grande Valse Brillante.
Is there any kind of documentary out there that explains how exactly people made these soundtracks?