The present study investigated the effect of background luminance on the self-reported valence ratings of auditory stimuli, as suggested by some earlier work. A secondary aim was to better characterise the effect of auditory valence on pupillary responses, on which the literature is inconsistent. Participants were randomly presented with sounds of different valence categories (negative, neutral, and positive) obtained from the IADS-E database. At the same time, the background luminance of the computer screen (in blue hue) was manipulated across three levels (i.e., low, medium, and high), with pupillometry confirming the expected strong effect of luminance on pupil size. Participants were asked to rate the valence of the presented sound under these different luminance levels. On a behavioural level, we found evidence for an effect of background luminance on the self-reported valence rating, with generally more positive ratings as background luminance increased. Turning to valence effects on pupil size, irrespective of background luminance, interestingly, we observed that pupils were smallest in the positive valence and the largest in negative valence condition, with neutral valence in between. In sum, the present findings provide evidence concerning a relationship between luminance perception (and hence pupil size) and self-reported valence of auditory stimuli, indicating a possible cross-modal interaction of auditory valence processing with completely task-irrelevant visual background luminance. We furthermore discuss the potential for future applications of the current findings in the clinical field.