Bone conduction headphones make sound by vibrating the bones near the ears, leaving one's ears open to hear ambient sound. I was able to test a set from Aftershokz. The short review is that they work as advertised but have some implementation problems that make them hard to use for bike touring.
The full review provides much more depth, including descriptions of the headphones, the good, bad and ugly parts, and recommendations.
The Aftershokz web site is www.aftershokz.com .