Pink noise or something like a drum track. Music helps in the sense that you can more readily distinguish strain and tonal change. I usually do a progressive ramp up. Initially start out listening and turn up the volume up to a level that just starts to show signs of strain/tonal change(brightening). Back off a tiny bit. Take the earphones off and let them sit playing for an hour. After the hour's up, put them back on and readjust the volume. Turn it up to where it just starts straining again. The volume level should be a little higher this time before it happens. Back it off just below this straining point and take off the earphones again. Wait an hour again. Put them back on and adjust the volume again. Keep repeating this until you get no additional change in volume before straining. This is effectively the mechanical limit of the earphones and they are largely broken in. the intervals don't really need to be an hour. You just slowly ramp up the volume to keep up with the loosening of the earphone and keeping it near its limit as it keeps moving more freely from use. The process is pretty short really. 100hours, 200hours, 500hours, no. 3, 5, 8 hours sure. If you listen to them at 25% for 100 hours, they'll never be broken in. The process does require you to stretch them out so to speak. You don't stress them, so you always stay below any sign of strain or tonal change which can indicate the hardware fighting against the movement. Not being careful about this can potentially wreck them.
Also this mainly pertains to dynamic drivers. Balanced armatures don't really have a break in process. Any change happens in a matter of seconds.
