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.
Unfortunately, there are currently no dumps of the media files available. However, there is a page that explains how to reuse the content outside of Wikimedia.
As an alternative, there is Forvo. Their audio files are licensed under the more restrictive (and not entirely open-data-compliant) CC BY-NC-SA license. On the other hand, they do offer an API.
Take a look at Project SHTOOKA
Maybe the Macmillan Dictionary API can help you
2It is not open data.– Nicolas Raoul ♦Dec 3, 2018 at 7:20
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.