Don't know about audio. (I like listening on long car journeys too!) but this was said about the Oxford Readers' Companion to the Brontes
Full review
The Oxford Companion to The Brontes is an extremely useful sourcebook of all things related to the Bronte family.
It is, in effect, an encyclopaedia on the Bronte family, and is clearly very well researched and very well written. It is also illustrated with copies of drawings by Bramwell Bronte.
It gives details of the family not found in other books on the family that I have seen. It reveals exactly how it was that Patrick Bronte came to take holy orders in the Church of England, where he trained and it even touches on Patrick Bronte's time at Christ Church, Wellington, Shropshire. A curious omission in many books on the Bronte family, as it was in Wellington that Patrick Bronte had a volume of poems published, Cottage Poems.
It also carries great detail about the plays that the Bronte children would stage, known as the Glass Town and Angrian Saga, set in fictitious states and using their toy soldiers as the main characters. All of the children wrote scripts, but only those written by Charlotte and Bramwell survive, those created by Emily and Anne are lost. At least at the moment.
The book details the real people that had influence (some seemingly rather peripheral, others of more importance) on the Bronte family and their writings, and who had influences on the name of the family after their deaths.
The book also details fictional characters from not only the novels but also from the plays they wrote as children. Indeed, there is a special section of the book devoted to Juvenilia of the Brontes, detailing there childhood literary and artistic accomplishments. Which were many, it must be said.