1. A speech search method performed by a display device, the method comprising: outputting media data by the display device; receiving, by the display device, a speech search command for additional data about the outputted media data from a user; determining, by the display device, whether the speech search command includes a query term that is immediately searchable by the display device; if the speech search command does include the query term that is immediately searchable by the display device, performing by the display device a search for the additional data using the query term that is immediately searchable; if the speech search command does not include the query term that is immediately searchable by the display device, matching by the display device a portion of the speech command with a query term extracted from audio data of a predetermined amount of the media data that is buffered by the display device prior to a time that the speech search command was received; and performing, by the display device, the search for the additional data using the extracted query term.