Originally Posted by California Vehicle Code
Wearing of Headsets or Earplugs
27400. A person operating a motor vehicle or bicycle may not wear a headset covering, or earplugs in, both ears.
So you can wear
a earphone.
But whether you do or don't wear earphones,
DO get some type of mirror. We teach people in driver's education to use a mirror, checking it every few seconds. If its important for automotive vehicles, that don't need to constantly worry about rear-ending, how much more so for bikes? Don't depend on you r ears. Newer cars, electric cars, hybrid cars, other bikers -- all can be very quiet.