Neuromodulatory systems, like the serotonergic system, innervate both sensory brain regions and regions involved in social behavior, making them likely candidates for representing social information in sensory systems. In an auditory midbrain nucleus (inferior colliculus; IC) of male house mice (Mus musculus), levels of serotonin are sensitive to both auditory and social stimuli. Serotonin in the IC of males increases when they are with a female partner, but the degree of that increase negatively correlates with the rejection behaviors, including the number of broadband vocalizations (BBVs) that females exhibit. Therefore, we hypothesize that serotonin in the IC of males encodes female receptivity during sexual interactions. To directly test the relationship between serotonin in the IC and perception of rejection BBVs, we infused a control (aCSF) or serotonin-releasing (fenfluramine) solution into the IC of males before BBV playback. Both control males and males with increased IC serotonin reduced ultrasonic vocalization (USV) production to BBV playback. However, fenfluramine-injected males produced fewer USV syllable types and had a more pronounced recovery after the end of BBV playback when compared to control males. Fenfluramine was also injected systemically into males prior to BBV playback to assess the role of a more widespread serotonergic increase. Fenfluramine depressed baseline vocalizations to the extent that further suppression by BBV playback could not be detected. Therefore, local increased IC serotonin does not change immediate USV response to female rejection vocalizations but may impact USV syllable repertoire and recovery, and systemic and local fenfluramine- induced increases in serotonin have markedly different effects.