Do we all hear the world in the same way? Recent auditory illusions involving speech or musical sounds tantalizingly suggest that the answer may be an unexpected “no”. Here, leveraging novel behavioural paradigms, physiological recordings, and causal interventions, we wish to reveal supra-threshold inter-individual differences in audition (IDAs) and provide tools to characterize them. Several novel behavioural tasks will be investigated. The paradigms will be intentionally varied, using speech sounds, musical sounds, or complex stochastic sounds, for tasks involving word reports, pitch shift judgements, spatial location judgments. Biases will be measured in the laboratory, but also on large cohorts of listeners recruited through crowd-sourcing platforms online. We will then extend the study of IDAs to memory, by asking whether, for a same sound, the content of the memory trace is common to all listeners or not. A computational modeling approach based will further question whether idiosyncratic perceptual biases represent an adaptive functional match to the information present in a listener's acoustic environment. Finally, we will seek physiological correlates of individual biases, as well as intervention techniques to modulate them, based on the hypothesis that memory and perceptual IDAs are closely intertwined. This will allow us to answer key questions: are biases linked to specific sounds, or are they instead characteristic of an individual listener? Are inter-individual differences simple quirks of auditory processing, or do they reflect a functional adaptation to each listener’s environment fine-tuned through learning? Can they be modulated through exposure? This project will be, to the best of our knowledge, the first large-scale investigation of IDAs, leveraging the complementarity of in-lab and online experiments to probe a much more diverse cohort of listeners than previously attempted. We will develop a completely novel behavioural assay, probing various aspects of sound relevant to speech and music perception. We will also provide a first physiological correlate of IDAs using a low-cost tool, pupillometry. Finally, our ambitious goal of modulating individual biases through intervention has broad translational appeal for the future of personalized assistive hearing devices.