Bike bells are useful for me most weekends to let me know there's a bike soon to overtake me while I'm skating.

Headphones on folks while they're out walking is ridiculous and antisocial and if they get hit because they didn't hear a bell then they had it coming. I only use a single earbud at a time so I don't lose my situational awareness entirely, but even that can still wash out the rest of the world noise pretty well.