I think there would be a limit to the "softening", like when you go for a run, your legs loosen up after the first mile or so. But the way I look at it, the coil's ability to create a magnetic field will not be changed by any slight softening of the wire. What theoretically could change is the wire's movement due to that field, which would then change the diaphram's movement. I don't see much other reason why a speaker's sound would change after playing it for any period of time, and this reason doesn't really do it for me. It is clear though that you can tear the diaphram if you "crank it." But I think for any well built speaker, if you don't abuse the rating (and your ears) you should be ok.
