Firstly, it's a language thing.
B&N probably thinks of it as 'audiobooks' or audioCDs'.
So if you search for something that is different from the language they use, it won't show up.
B&N doesn't have an 'intelligent' association that says - if user searches for downloadable books or downloadable mp3s, it probably means audiobooks.
Secondly, I've found that the search is very erratic. Best bet is to go to the specific section i.e. http://www.barnesandnoble.com/s?SRT=SA&FMT=A1&store=BOOK&aref=1518
And then use the categories on the left and the sorting criteria to look for books.
Thirdly, try book name + audiobook OR author name + audiobook. That should work.
Strange behavior overall! I tried to purchase an e-book via B&N, that clearly stated it was compatible in Nook Tablet format. The book arrived on my NT, and it states not compatible with this device. Huh? I emailed B&N customer service, first got the auto-reply, then got a reply stating they are too busy to respond. Huh? (I disputed the charge via my credit card, as I am not paying for what I did not receive.) This puts B&N either very busy (good) for the holiday shoppers, or in the same poor customer service category as the cable, phone, Microsoft, HP, tech companies that clearly do not want to offer human or rapid e-customer service. I love my NT, yet will probably not buy another. After one year, the wifi signal is slow and I have trouble connecting at both public wifi and via password wifi (I've posted about this before, outgoing servers are not playing nicely, esp. TWC roadrunner & legacy bell south. I went to B&N customer service store, before my NT was just one year old, and they offered to replace it with a refurbirshed, pre-owned NT (take on someone else's problems?). I like B&N and very much want them to survive, yet do they care about their customers? I think they should sell off the NOOK e-reader and mini-tablet to those who know how to do it and then we can all buy something else next time. I had great hopes. Thank you for e-listening!