No, you're not aware of what you hear when you're sleeping. More likely you'll cause yourself to sleep restlessly or wake yourself and function more poorly the next day for it.
If you have a decent tape recorder record your teacher and play back the lecture a bit at a time to break down what they're saying. Teachers are very deliberate in what they say in front of a classroom.

If you want to improve retention of studied material eat a ballanced diet and sleep properly. Don't eat heavy meals before studying. Take short breaks every hour to 90 mintues where you get fresh air and excercise to keep your mind fresh and able to focus, even if you're just walking around your block and rehashing what you were just going over in your mind. If you're studying written material make sure the page is brightly lit. You're better able to remember charts or other illustrations if they are clearly illuminated.

ohhhhhh damn..........really?? i was like really excited to do it b.c i remeber seeing somthing that had a person listening to a tape that helped cravings and losing weight....... so thanks for answerhing though!

Well, having something playing whilst sleeping, really does not help.
Though, studying before going to sleep, does tend to help.
Try reading the chapter once or twice; yes that may seem obscured, but it works. And after you've read it, go over what you had just read. But, my study-methods consist of reading the chapter once, possibly more, studying randomly, thinking about what I've read throughout a day, and looking through what it is I've got to know before falling asleep.

And, if you've got something like Terminology,
try to come up with things that have something to do with the word. I remembered civil law by ' civil-converse.' Criminal, 'Criminal-crazy.'

haha, me and studying dont do well. I get veryy unfocused, and cant study for longer then 5 minutes. I always need to do something fun. But yeah studying be4 sleeping...okay! i ha ve 4 more tests this week. ill try that...! thnak you!!!!

this may see weird, but make sure you are eating right. take a break and go back to studying. sometimes if i read things twice it helps me because i loose interest fast also.