The container. MKV is the preferred format for dual audio because it can hold virtually unlimited audio tracks, subtitles (SRT/PGS), and chapters in one file. Unlike MP4, which often struggles with multiple audios, MKV handles them seamlessly.
Specifically mentions the inclusion of the Hindi dubbed track. Note: Some files might say "Hindi + English," but here it is explicitly labeled. Nancy Drew 2007 Dual Audio Hindi 720p WEB-DL.mkv