The two first years of human life constitute a critical period for the emergence of robust speech encoding. Event-related brain potentials as well as the Frequency Following Response can be used to decipher how speech sounds are encoded along the auditory pathway at the cortical and subcortical levels. Previous electrophysiological studies have shown that experience can trigger neural plasticity within these two structures resulting in increased quality of speech sound encoding. However, the sensitivity to plastic experience-dependant mechanisms of each structure during early infancy remain largely unknown. Here, we gathered simultaneous cortical and subcortical brain responses to synthetic speech sounds in 6- to 24 months-old infants. We developed an innovative experimental paradigm that allowed to analyze both ERPs and FFRs in young infants and thus offering a new tool to obtain comprehensive developmental auditory profiles. Our results show how cortical and subcortical responses to speech sounds interact during early human development. We expect these findings to session pave the way for an integrative view of early developmental plasticity mechanisms taking place along the human auditory pathway and accounting for the gradual emergence of native speech sound encoding abilities.