I think you should decide for yourself what proportion the sense of hearing as opposed to say, the sense of sight, you use to detect cars approaching, most importantly from the rear as you weave left and right across the shoulder line, but also from the sides and front, as you subconsciously assess how much space there would be left for you if an oncoming car and an overtaking one cross right where you happen to be. You might want to think about whether you subconsciously assess whether what is coming up behind you is a motorcycle or a logging truck so you can take the two to three second precaution of getting even further over to the right with the latter - or even completely off the road.
It takes only one car to kill a cyclist and death can occur in both heavy and light traffic. In light traffic, drivers may not be looking for obstacles on the road - maybe because they are distracted by the radio, cell phone, a lit cigarette or doobie, an alcoholic buzz, a struggle with an infant, a conversation with a passenger, chewing their gum too fast - or even by wearing headphones.
If the quiet of the wilderness is just too quiet, you should consider a bicycle stereo, never headphones:
http://kumo.swcp.com/byco/Riding with headphones is illegal in many states, including California.
