I used to be an aerobics instructor and understand the motivational potential of music during a workout. However, on the road as well as off-road, I want to be as fully engaged with the ride as possible and never desire audible distractions. A part of getting outdoors is to connect with what's around me. Without the distraction of music (or a rubbing derailleur) I'm in contact with the other sounds including approaching cars. You have to be blind - or totally distracted - to see oblivious riders putting themselves in danger because they can't hear. Besides, having both ears blocked is illegal (at least in California) when riding.