Speech perception is the process by which the sounds of language are heard, interpreted and understood. The study of speech perception is closely linked to the fields of phonology and phonetics in linguistics and cognitive psychology and perception in psychology. Research in speech perception seeks to understand how human listeners recognize speech sounds and use this information to understand spoken language. Source: Wikipedia
There are (at least) six current (competing) theories which seek to explain how humans manage to interpret language so efficiently - and how they seem to have the innate ability to do so.
The main theories are :
The Motor Theory, Exemplar theory, Acoustic Landmark theory, the Fuzzy Logic model, the Speech Mode hypothesis, and the Direct Realist theory.
All of the above are detailed, here, at Wikipedia