1. A speech search method performed by a display device, the method comprising: outputting media data including audio data; receiving a speech search command for additional data about the outputted media data from a user, the speech search command including at least one query word; determining whether the at least one query word matches a query term that is full and searchable; when the at least one query word matches the query term that is full and searchable, performing a search for the additional data using the query term; and when the at least one query word does not match the query term that is full and searchable, determining the query term from a predetermined amount of the audio data prior to receiving the speech search command and performing the search for the additional data using the query term.