In this photo the guy is holding his headphones in his teeth as he stands in line. This person needs a way to listen to music without having his earbuds in his ears. Maybe the chord could send vibrations through his mouth (similar to the toothbrush idea) so he could hear the music while he has his earbuds in his mouth.
In this photo, the man is listening to music while he rides his bike. He could use a light weight and attachable mp3 player so he could ride without a cumbersome mp3 player. He could also use a way to control the mp3 player as a part of the bike.
This woman is running while listening to music, like the bike rider she needs something light and easy to control ideally without looking at the interface. This means something not overly complex and easy to attach to something like your arm. In this case it might be good to stay away from a touch screen interface and keep things simple.
This man is walking outside and in and out of a sunlight. It might be beneficial for the interface of his mp3 player to adjust to light conditions. If he is in shade the display brightens if he is in sun the display becomes darker, this would improve readability of the interface and improve user experience.
This woman is wearing a backpack and walking while listening to music. She could use a way to hold her mp3 player in her backpack. Perhaps, earbuds could be built into the straps, and the back pack could be an mp3 player itself.