Research on second language (L2) speech learning suggests that incidental perception training can lead to the establishment of non-native phonological categories. The present study contributes to this line of enquiry by investigating how this training is mediated by individual differences in working memory capacity and domain-general auditory processing abilities. In our study, 130 native British English speakers without prior knowledge of Portuguese were randomly assigned to trained or untrained conditions. All participants completed a visual digit span task and an auditory processing test battery. We observed improvements from pretest to post-test in production only, but since both groups improved, these gains cannot be attributed to the incidental perception training. The analysis of the ID measures further confirms the important role played by auditory processing abilities in L2 speech learning. However, more research is needed to better understand the role of incidental perception training and the mediating role of cognitive aptitudes.