Oh, my.
It turns out that there are quite a few Captivating History titles of which the ebook is free and the audio narration is $5.95.
If that's of interest to you, I recommend that you do
a search of Captivating History titles, then do a search for "Whispersync" and see what you come up with. Make sure, if you want the best deal, that you check to see that the ebook is $0.00.
I remember some of these titles being free and being posted on MobileRead recently. However, at the time, I hadn't made any commitment to try to ferret out Whispersync deals, so I likely just ignored any mention of WS audio narration.
If you think that you might be interested in one of the ebooks/audios, I suggest that you check Amazon and GoodReads for ratings of the ebooks, and Audible for the audio portion.