Go check this out:
http://www.megaupload.com/?d=BOAXXP38
If you really want to excel in American Accent, the shortest path is to start with the very basics. (Though there must exist exceptions, this is true for most people.)
The following material has been helpful to me from time to time:
http://blog.wenxuecity.com/blogview.php?date=200911&postID=7818
And this old rave might somewhat make a little sense as well:
http://web.wenxuecity.com/BBSView.php?SubID=mysj_best&MsgID=13240
I believe your voice and tone are not the same when you talk normally and when you record the above clip. REEEEEELAX!
Good luck.
NQ