A method of playing audio content to a viewer in synchronisation with a video content. The method comprises receiving and digitising an ultrasonic signal comprising ultrasonic synchronisation signal(s) encoding a respective timecode through modulation of ultrasonic carrier signal(s), a first ultrasonic carrier signal having been amplitude modulated to encode logical “1” or “0” in data-carrying segments, each data-carrying segments being followed by non-data-carrying segments in which the amplitude of the ultrasonic carrier signal does not exceed a predetermined threshold. A digitised ultrasonic synchronisation signal is processed to identify the data-carrying segments, and the corresponding timecode is extracted from the data-carrying segments. A stored audio content is played back from a playback point determined based on the timecode.