The source of the phonetic transcriptions is Oxford Advanced Learner's Dictionary (3rd ed.), available from the Oxford Text Archive (http://www.ota.ahds.ac.uk). This is an English pronunciation dictionary which contains 70,647 items with RP (Received Pronunciation) transcriptions.

The above line is claimed by WikSpeak, http://wikspeak.sourceforge.net/ in its installation file, wikspeak1.1\wikspeak\transcriptions\transcriptions.txt

I just convert the dicchar file into json.