If you want a physical/offline archive, seek a properly muxed MKV file from a trusted private tracker (ensuring the Hindi AC3 track is perfectly synced to the 720p Blu-ray source).
You might ask: In an age of 4K HDR, why search for "Slumdog Millionaire dual audio Hindi eng 720p"? There are practical and aesthetic reasons. Slumdog Millionaire dual audio hindi eng 720p