The neural underpinnings of congenital amusia, an innate neurogenetic disorder of musical pitch processing, are not well understood. Previous studies suggest that amusia primarily impairs attentive processing (P300) of small pitch deviations in music, leaving pre-attentive pitch processing (mismatch negativity or MMN) more or less intact. However, it remains unknown whether the same neuro-dynamic mechanism of deficiency underlies pitch processing in speech, where amusics also often show impairment behaviorally. The current study examined how lexical tones are processed in pre-attentive (MMN) and attentive (P300) conditions in 24 Cantonese-speaking amusics and 24 matched controls. At the pre-attentive level, Cantonese-speaking amusics exhibited normal MMN responses to lexical tone changes, even for tone pairs with small pitch differences (mid level vs. low level tone; high rising vs. low rising tone). However, at the attentive level, amusics exhibited reduced P3a amplitude for all tone pairs, and further reduced P3b amplitude for tone pairs with small pitch differences. These results suggest that the amusic brain detects tone changes normally pre-attentively, but shows impairment in consciously detecting the same tone differences. Consistent with previous findings in nonspeech pitch processing, this finding provides support for a domain-general neuro-dynamic mechanism of deficient attentive pitch processing in amusia.