If you're looking for a specific dual audio version (e.g., English with subtitles in another language), you might find it more readily available on certain platforms or through specific releases of the movie.
The dual audio download of "I Am Legend" is available on various platforms, including:
A progress bar appeared. It crawled at first, then leaped forward. 10%... 45%... 90%. As the file finalized, Mark checked the file size: 2.4GB. That felt right. He didn’t want a compressed, pixelated mess where Robert Neville’s dog, Sam, looked like a brown smudge.
Are you more interested in the or the deeper, philosophical alternate ending of the movie?