I don't ride with earphones. I have an mp3 player with a speaker on it. It is a zen stone. I like being able to hear my music, but also listen out for cars, peds, and other cyclist.

Usually I can hear a cyclist coming up on me...this is important because my road in filled with rocks here, screws there, nuts and bolts...dirty diapers..glass..etc. So I am constantly moving my wheel to get out of the way of some of this stuf...oh and potholes...
some things i can do a sharp little move that barely pushes me off my line, but other times I really have to make a move

Now if another cyclist was coming up and didn't say on your left or good morning etc..we would ram into each other. Because I am usually not looking behind me in the bike lane. Or sometimes things come upon you suddenly..and listening out for a car to know if you need to brake or you can make that slight move is something that comes in handy for me during my commute.