The present invention provides an audio codec for converting digital audio signals to analogue audio signals, the audio codec comprising: at least two digital audio bus interfaces (40, 41) for coupling to respective digital audio buses or bus sections; digital- to-analogue conversion means (13, 14) for converting digital audio signals received through first and second digital audio bus interfaces to analogue audio signals, for output to an audio transducer (10); adding means (15a, 15b) for adding together digital audio signals received through the first and second digital audio bus interfaces to form a combined digital audio signal; and means to route the combined signal to one of the first and second digital audio bus interfaces of the audio codec, or to a third digital audio bus interface of the audio codec, for transmission on a digital audio bus or bus section.