RIDING a bicycle with headphones occupying both ears is not just dangerous, but also illegal in New York and many other states. And while it’s O.K. to ride with just one ear plugged in, that’s not always very pleasant. Gadget makers have devised a solution: compact speakers for MP3 players that are meant to be attached to a bicycle, either on the handlebars or elsewhere. The rider selects the songs, attaches the music player to the speakers, then listens and rides with ears wide open.