It depends on which player you use. In VLC, open the video, right click the video area, go to Audio -> Audio Track. This will be similar in most players. If there's different audio tracks, you'll find them there. Otherwise you'll have to download the full audio of the movie, then merge it with the movie you have... and where will you find such audio? Only in the movie itself, so you're gonna have to find a new copy if this one does not have an English audio track. Another option is finding the subtitles and pretending it's a foreign movie.