As the covert and clandestine practices of states multiplied in the twentieth century, so did these practices’ footprint in public life. This footprint is not just visual and material but sonic and aural, sounding the ‘secret state’ into being and suggesting ways of ‘listening in’ on it. Using multisensory methodology, this article examines Careless Talk Costs Lives, a UK Second World War propaganda campaign instructing citizens on how to practice discreet speech and listening in defence against ‘fifth columnist’ spies. This campaign reproduced the British secret state in the everyday: it represented sensitive operations as weaving in and out of citizens’ lives through imprudent chatter about ‘hush-hush’ activities and sounds you shouldn't overhear. The paradox at the campaign's heart – of revealing to people the kind of things they shouldn't say or listen to – made the secret state and its international operations a public phenomenon. Secret sounds therefore became entangled within productions of social difference, from class inequalities to German racialisation. Sound and listening, however, are unwieldy phenomena. This sonic life of the secret state risked undermining political legitimacy, while turning public space and idyllic environments into deceptive soundscapes – for international espionage, it seemed, sounded like ordinary life.