That's a good point. I'd like to see some official numbers on that.
That said, obviously it must be less than GPS mode, since it more than halves it. My expectation for 5S would be something little over 7 hours:
Drain on battery in watch only mode is about .5%/hour, drain on battery in GPS mode is about 9%/hour, and drain on battery in GPS+music mode is about 22%/hour. Which leads me to believe music only will eat about 13.5%/hour.
