Headphones solve a lot of problems, but they also create new ones. By blocking our ear canals we become less aware of the world around us. Runners and cyclists might not hear a car approaching. In a work environment a co-worker could struggle to get your attention (maybe that was the idea). In high noise environments we crank up the volume to dangerous levels. Fortunately, bone conduction has become more accessible (thanks ex-militar tech). It sends sound waves directly to the inner ear. This allows for situational awareness as the ear canals are left open. For daily use and athletes the AfterShokz Trekz Titanium seem promising based on a detailed review over at The Verge. Want to roll your own? Adafruit sells a pretty sweet bone conductor transducer for exactly this purpose.