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 based on pronunciation patterns of the user; when the at least one query word matches the query term that is full and searchable and the pronunciation pattern is a first pattern, 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 and the pronunciation pattern is a second pattern, 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, wherein the first pattern means at least one of that the user's accent is clear and that there is no mumbling in the user's pronunciation, and wherein the second pattern means at least one of that the user's accent is unclear and that there is mumbling in the user's pronunciation.