According to kuhl's research, at birth, infants are "citizens of the world," able to recognize sounds regardless of the syllables' language. at what age do infants get better at perceiving the changes in sounds in their own language

I believe the answer is 8-12 months.
It's regarded as an important milestone for infants' communicating development. Studies shown that due to this ability to perceive change, the caretaker that hear the cry the most usually able to distinguish the thing that the infants one by hearing the type of cry that they made.