I don't really buy into the whole 'burn in' idea. To me, it makes sense that it's not that the sound gets better, you just become accustomed to the sound. You acquire a taste for it. Maybe in a similar way to someone who doesn't like a certain food, can develop a taste for it after a while. Although, that doesn't really explain why someone who has a pair of headphones, then tries a new pair of the same headphones and finds that the sound is different.
There might be some change in the headphones or speakers, but the majority is down to the user. The sound a speaker or headphone produces cannot radically change unless it has been physically altered, whereas our likes and dislikes change constantly.