Is there any downloadable open database of English words pronunciation in audio format?
Wikimedia Commons currently offers more than 20.000 sound files with English pronunciation, around 1.500 of those with British English pronunciation. All of them are published under an open license.
Take a look at Project SHTOOKA
Maybe the Macmillan Dictionary API can help you
The dump here contains nearly all pronunciation data used on the English wiktionary. So the files are from a combination of sources like project SHTOOKA, Lingualibre, etc.