I prefer headphones tuned low over external speakers. Number one thing I hate on a ride is listening to someone elses idea of music, I have a wide taste in music, but some of it is just plain noise.
With headphones I normally have the volume turned up just enough to hear, they also cut down on the windnoise so I actually hear vehicles easier, the whoosh of the wind easily covers the whoosh of an approaching car, if I don't hear the wind I can hear the car while it's farther away.

ken.