I'm looking for a website that would propose free dictations in English, so that I could listen and then type what I hear and at the end to see if I made any mistakes. I was searching in the Internet, but I didn't find anything close enough. May be someone knows and wouldn't mind to share ? Thanks!
This meta post on ELL lists many resources, one of which relates to pronunciations. The other resources are quite useful as well.
The Writer's Almanac is a daily radio segment/podcast which features Garrison Keillor in 5 minutes of talk and poetry. There is not a literal transcript for the entire program, but there is a "set of notes", so to speak, which can used to help interpret what is said, and there is an exact copy of the poem featured in each segment.
Keillor is a good source for pronunciations and diction as he has a clear and near-perfect Midwestern accent, and I only very rarely detect what I would consider a mispronunciation. (Though some episodes in the summer of 2013 were done by poet Billy Collins, who notably cannot pronounce "poem" correctly.)