The association between vowels and valence is known as valence sound symbolism. For example, the vowel /i/ (as in English "meet") is more associated with positive valence than the vowel /o/ (as in French "rose"), and this phenomenon has been extensively studied using explicit and deliberate methods. This study investigates valence sound symbolism through an implicit paradigm adapted from Vainio (2021). The aim is to determine whether valence sound symbolism, like size and shape sound symbolism, operates automatically. Participants (N = 42) classified valenced facial expressions by articulating the target vowels. Specifically, in the congruent blocks, participants were instructed to say /i/ for happy and /o/ for angry faces, and in the incongruent blocks, to say /i/ for angry and /o/ for happy faces. We found significantly faster response times in congruent blocks than in the incongruent blocks. The analysis of acoustic parameters, particularly the fundamental frequency (f0), revealed a significant interaction between congruency and vowel type. Specifically, /i/ was pronounced with higher pitch in the congruent (happy face) than in the incongruent blocks (angry face), whereas /o/ showed the opposite pattern, with higher pitch in the incongruent (happy face) than in the congruent blocks (angry face), indicating that participants' pitch was modulated by the match or mismatch between the vowels and the valence of the faces. In sum, the present results indicate that the association between vowels and valence operates automatically, thereby contributing to our knowledge of cognitive processing during valence sound symbolism.