An English audio track for I Saw the Devil (2010) exists and is available through official digital platforms as a "Dubbed" version. While there is no standalone official "audio track download" service, you can access or download the dubbed movie legally for offline viewing via several major retailers. Legal Ways to Get the English Audio

If you choose to move forward with downloading, ensure your file has these three things:

Several reputable platforms offer I Saw The Devil with an English audio track.

Physical discs include a where you can toggle between the original Korean audio (with English subtitles) and the English Dubbed audio track.