In 2003, 9-month-old American babies heard Mandarin during twelve short sessions. Those who played with a live speaker learned to tell apart Mandarin sounds. Those who watched the very same sessions on video, or only heard the audio, learned nothing measurable. Social interaction seems to be key to learning speech sounds.
In Kuşköy, a mountain village in northern Turkey, people talk across valleys by whistling Turkish. A 2015 study showed that whistled Turkish draws on both hemispheres fairly evenly, whereas spoken Turkish relies mostly on the left. The physical form of the signal, it turns out, matters to the brain.
Speech has no gaps between words. In 1996, researchers played 8-month-old babies a continuous stream of made-up syllables for just two minutes. The infants picked out the 'words' by tracking which syllables tend to follow one another. That's probability calculation, done without a calculator or any visible effort.