I have known about Randall’s ESL Cyber listening lab for a long time, but somehow I forgot to put a link to it in the study list of useful websites on the right-hand side of this page.
Anyway, better late than never I always say 🙂
On this site you will see that audio texts are graded into three different levels. They cover all different kinds of topics with a range of different accents, but a lot of American stuff, I guess. The listening tasks often have pre-listening activities to start you thinking about the topic and comprehension questions to check your understanding. There are quizzes and academic English tasks, and also there are some vocabulary building exercises and further tips for students on how to study effectively in your own time.
Have a look at the site by clicking on the link above or from the study box on the right. Let me know if you have any success, or, if you have any comments, please post them. Cheers!