This can't be done... The audio for the file is encoded along with the video, so you'd have a pretty hard time in doing this.
The subtitles are just something that runs alongside the file, its not an essential part of it.
EDITThis can't easily be done
My mistake, I meant to write that ^^
I'm no expert on A/V but I'm guessing you would have to re-encode the file with the new Endligh audio... Which, I would assume, would be very difficult to find.
But yeah, the word "downloaded" and "film" in the same sentence does sound dodgy.