Answered for language learning in general. No specific language was set for this question, so the findings are drawn from general second-language research rather than one language’s literature.
The sources don't support the idea that people can learn a language from scratch just by playing recordings while asleep. What the evidence does show is that sleep helps strengthen and organize new words or skills that were already studied while awake — sleeping soon after study sessions seems to improve retention, and in lab settings, playing sound cues tied to already-learned material during deep sleep can give a small memory boost, but this is reinforcement of prior learning, not learning brand-new content unconsciously.
The clearest language-related evidence shows sleep helps cement new vocabulary that a person already studied before falling asleep — it does not show that people can absorb brand-new words or language content purely by having it played to them while unconscious.1,10
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Across several studies of children and adults, people who slept (even a short nap) shortly after learning new vocabulary retained and understood it better than people who stayed awake for a similar stretch of time.1,3,10
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Some experiments found that quietly replaying a sound linked to something already learned (like a matching object sound) while a person is in deep sleep can improve memory for that specific material afterward — but this only worked for weak or low-priority memories, and only when the cue was tied to prior learning, not to brand-new information.2,3,4,7,9
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The benefit of sleep for memory seems tied to specific sleep stages and brain-wave patterns (like slow-wave sleep and sleep spindles), not simply to time spent unconscious — and these details also determine whether sound cues played during sleep actually help or do nothing.2,5,6,7,8,9
In plain terms