I enjoy making short animations and consider the sound track a key element. Sometimes its easier to let the viewer's imagination create the scene based on sound effects. Here is one of the best examples I've ever seen (caution: violence, language):

https://www.youtube.com/watch?v=UMWm03yiGsg

I have a large, indexed sound effects collection from Digital Juice, but it doesn't have everything. So maybe a little retro approach is needed on occasion:

https://www.youtube.com/watch?v=hZ43UC5tIOY

A more recent, and very funny example:

https://www.youtube.com/watch?v=SUZaGjfVLCM

Here is an article on lots of free classic radio dramas:

http://www.usatoday.com/story/tech/columnist/saltzman/2016/03/25/webs-best-kept-secret-thousands-free-classic-radio-dramas/82204502/

How awesome! Thanks for posting these awesome links.

Hope Kumor

Editor-in-chief of Renderosity Magazine