Im a bit backwards. When I toured this summer I had some speakers that didnt require batteries that I put in a bag that mounted to the stem/top tube junction. When riding country roads, I listened to music using those.

When in the city, I rode with headphones. i

I know people may call be crazy but my reasons are my own. I live in a large city and try to be hyper-observant when riding. So, I mainly rely on sight instead of sound, scanning several second ahead and to the sides of me. The way I figure it, if someone is going to hit me from behind, there is not much I can do about it. If they are close enough for me to hear them in a city of 1 million plus, they are already too close.