"To this day, when I work out I usually listen to ballads." - Rollins.
I recently read something about listening to one song on repeat for your entire workout to help focus, kinda like how you hear about geniuses wearing the same outfit to eliminate unnecessary decision making. I gave it a shot, didn't notice anything special other than surprisingly getting a little irritated by the monotony after the 4th go'round. So, kinda counterproductive.
This summer, I trained in the backyard and sometimes went with no music, just listened to the birds, cicadas, cars, and regular suburban noise. That was actually a nice change of pace.