Well, I can't hear shit. Ever watched those boring commute videos people film on their bikes, where it's all WHHSSSSSHHHHHHBRRRRRSSSHHHHHHHH? Above 15mph or so, that's all I hear.
I make up for that by checking behind every few seconds, which I never see anyone else do by the way. I have music at a listenable volume so when not moving quickly, I can still hear the important stuff - e.g. when I'm at the front at the lights, drivers behind are wondering why I'm not going anywhere on green, just as they nearly get taken out by the ambulance coming through the junction.
As it is, I find riding without them demotivating, and my mind wanders off to thinking about all sorts of inane things like how to make it rain gravy (what would happen if you tried to seed clouds with a mix of potassium and Bisto?) and before you know it I've crashed into the back of someone.