What’s the source of the 5.1 audio for ESB? Is this a reconstruction like HairyHen did for SW or something else?
Pretty much. It’s the audio from the 93 laserdiscs (and a bass channel from the blurays) with all the necessary tweeks (levels, fixing a few missing sound effects, etc)
But keep in mind that the 70mm version, which would have contained the surround sound mix, is different in content so this is not a reconstruction so to speak. It’s more of a “what if the content of the 35mm stereo version was in surround sound?”