Barnes and Noble has not really developed many accessories for its line of tablets, but we should see a new wireless speaker very soon. The largest bookstore chain the USA has just had its first wireless Bluetooth speaker pass the FCC. This will allow you to take advantage of better audio capabilities when listening to music, audiobooks, or a myriad of other things with your Nook HD and Nook HD+.
There is no word on cost yet, but we should see a commercial release within the next two months. This is going to be a great addition for people who are not totally content with the built in speakers in their tablets. You may pay a premium for this, but if you listen to lots of audio, it may be worth it.