Could you please give me feedback on my pronunciation? (English)

Hello and thank you for reading my question!

I'm currently working on a research paper in American phonetics.
I want to improve my pronunciation because I need to sound good if I want to convince my examiners that my research paper is interesting.
My goal is to become an English teacher.

Could you please listen to my recording?
http://soundcloud.com/englishpractice88/practice11112012
There's nothing to download, the recording is hosted on a website similar to Youtube.

1. If I were an English speaker, where would I be from?
2. What words did I mispronounce?
3. How can I improve?
4. Overall, am I understandable?

I'd be more than happy if someone accepted to swap a couple of audio files with me, preferably someone from the U.S. (or Canada) so that I could work on my pronunciation with a native speaker.

Have a great day everyone!