In a now well-known experiment, they divided a group of monkeys into two groups. Both groups had a sheath with something brushing their fingers, and headphones putting out a rythm. Both groups received exactly the same stimuli at the same time. But one group was trained to notice subtle changes in the strokes their fingers were feeling, and the other group was trained to notice subtle changes in the rythm. The monkeys were rewarded for noticing what they were supposed to notice.
The monkeys that were trained to notice changes in rythm later on had noticable changes in their auditory cortex (part of brain that processes sound). Their auditory cortexes grew, while those that were trained to notice changes in strokes had growth in their somatosensory cortexes (part of brain that processes touch).
Neither groups experienced growth in the part of the brain that processed the stimuli they weren't supposed to notice, and remember, they all recieved the same stimuli at the same time. From this it is theorized that attention is necessary to grwoth in the brain, or in laymen's terms, just plain learning. Attention is the key to all major changes in the brain it seems. Of course a person while sleeping is either not even concious (no attention), or their attention is not under their control.
You can take a smart guy, and put him into a classroom, but if his attention's not on the subject being taught, perhaps he just had a major fight with his girlfriend or something, and he's thinking about that, all that information that's in that room isn't going into his head.
I'd say that listening to tapes while one is sleeping doesn't work from the evidence we've seen so far.