I often ride listening to music or ebooks when I am just out spinning. Only wear 1 side so I can hear runners, other cyclist, auto traffic etc.
Have had 3 or 4 crashes over the years with people listening to loud music and having no real understanding what is happening around them.
As an aside - I was reading an article in a magazine while at the dentist last year. An insurance company refused to pay out when a cyclist caused an accident, wearing ear buds and not being aware of surroundings. In the resultant court case the judge agreed with the insurance company that rider was not taking due care.