If a person hears or listens to a verse of sajdah being recited by a person who is asleep or insane, or by a child who does not recognise the verses of the Qur’an, then sajdah becomes obligatory for him. However, if he hears it from a gramophone or a tape recorder [or some other sound-playing device], then sajdah is not obligatory. The same applies [i.e. sajdah is not obligatory] to hearing it from a radio if it is pre-recorded.
However, if someone recites a verse of sajdah live on the radio and one listens to it live, then sajdah is obligatory.