The phonemic merger phenomena is characterized by the inability of distinguishing two acoustically different phonemes, such as /n/ and /l/. Previous studies suggested that the process that drives the phonemic merger is the lack of separated memory traces of phonemes. However, it is still unknown whether higher-level linguistic functions have an influence on the perception of merged phonemes. We designed a mismatch negativity (MMN) study to explore the influence of semantic information in the perception of the phonemic merger. The results showed that no robust MMN was elicited in an oddball stream of the syllables /niu2/ and /liu2/ in the group of merged speakers. We interpreted that the brain of the /n/-/l/ merged speakers had almost lost the ability of early auditory discrimination of the phonemes /n/ and /l/. Interestingly, the same merged speakers regained the ability of discriminating /n/ and /l/ when the oddball contrast was placed in an idiom context. All in all, our results indicate that a context with a meaning, such as an idiom, facilitates the early acoustic processing of merged phonemes. This finding also gives additional evidence that not only the bottom-up analysis of the acoustic space, but also the top-down modulation of the functional significance of sounds could contribute to the early auditory discrimination of phonemes.