This study addressed the extraction of long-term sound familiarity as a higher-order feature of complex, environmental sounds. Physically variable, familiar animal sounds and spectrotemporally matched, unfamiliar control sounds were presented. Participants ignored the acoustic stimuli. Infrequent deviant sounds violated the familiarity status established by the standard sounds, but no regularity on the basis of physical features. In the auditory event-related potential, deviants elicited a negative-going deflection over parietal scalp areas around 230 ms. This effect occurred for familiar deviants among unfamiliar standards and for unfamiliar deviants among familiar standards. The results indicate the establishment of an auditory regularity based on sound familiarity. This reflects the extraction of sound familiarity outside the focus of attention.