有哪些难度大点的英语听力材料，语速要快点的。

推荐你购买空中英语教室（Studio Classroom）的高级版《彭蒙惠英语》，内容范围比较广泛，涉及历史、文化、医学等方面，每集都就一个话题展开讨论，提高英语听力之余也可以学到不少东西。价格也不贵，42块有三四张光碟，每天听一篇（约25分钟），也差不多能听上一年。

STEP BY STEP后几本
外研社出的高级英语听力(Listen to this)

可以找一些你学校里的英语专业的朋友请教一下

I'm not sure what do you mean by basically understand CNN BBC VOA. Is it 50% , 60%or more? If you can understand up to 90% of CNN broadcast, then your English must be really something. In those three stations you mentioned, it is true that a lot of topics is about politics. But there are also some programs about history, culture, medicine, etc. And by the way, language study is a two way process. If you just listen and never have any chance to practice it at all, you'll forget it very soon. I suggest you practice you conversation with native speakers or you can call VOA live broadcast program. Call collect, it is free. Wish you good luck.....