The Dark Knight [2008-BDRip-H.264]-NewArtRiot
Dolby Digital AC3 640Kbit/s 5.1ch English
This is an additional audio pack. It does not contain any video, only one or more alternative audio streams that are intended to be combined with the video from an existing release.
AC3 files can be muxed in to a matroska container with the video from an existing release using mkvmerge GUI. Add the release video file and the additional audio file to the "Input Files" box. Choose the tracks you want (you might not want to include the original release audio). Then press "Start muxing" to create the file.