Speaker break-in time myth or reality tries to debunk the myth of audio speakers needing time in orders of 30-50 hours to get to their best audio properties after purchase.
I bought a new set of Sennheiser HD 215 a few weeks ago. I unpacked them and connected to my laptop. I was blown away by their performance, but something at how the bass frequencies were "muddy" and not clean confused me. Then I read in the manual that I should play some music on the headphones for at least 30 hours to break them in and only then they would get to their best audio characteristics. So I left the headphones on the laptop and left some music playing in them overnight and only came to them after at least 20 more hours of breaking-in. I put the same music on as I did the first time and I was not disappointed - the bass was much more clean and instruments in the bass area were finally as distinct as they should be at this level of headphones. I did not listen to the music during the break-in time, so my brain did not adopt to the sound of these headphones - they actually sounded better.
I do not really care that a distinguished loudspeaker engineer can not find an explanation of how this break-in happens, but that does not change the fact - it exists.
Update - A few more links:

In the end people end up playing sine waves and measuring frequency response thus oversimplifying the problem. It is well possible that non-sinusiodal sound is needed for proper break-in (as in sound with multiple different frequencies at once, or sound with harmonics, ...). It is well possible that the "sharpness" or "muddiness" that is reported by the audiophiles as the characteristic of a non-broken in sound system is not something that is measurable by frequency response - enharmonic dispositions, temporal dispositions and other distortions of the sound are easy to detect by a trained ear, but very hard measure with precision unless targeted. Also people ignore the real complexities of the materials that are used. We think that wood is simple and plain and understood material, but in reality wood is such a superior micro-composite material that it has properties that are still not understood by modern material science. And when we get cooperation of metallic parts, wood, chemicals and, most importantly, natural and complex vibration - predicting all effects of such vibration on the complex system, that is a simple speaker, is ... well ... complex. Audiophile ear can easily hear distortions that normal audio equipment simply can not measure with any confidence.

Discarding phenomena that one simply does not know how to measure or explain is just blindfolding yourself.

I was even more fascinated by the description of an simple non-scientific experiment showing break-in effect on audio cables. This was discarded outright as impossible, but it is actually much easier to understand. Cable is simply a thin piece of metal through which a stream of electrons is flowing. It is not too hard to imagine that these electrons traveling at light speed could impact any impurities or defects in the cable. Any such impacts would distort the analog signal going through the cable. Also after multiple weeks of electron bombardment it is not impossible to imagine such impurities or defects (or the material around them) being changed in a way to ease the flow of electrons (like wind erosion of rocks) thus breaking-in the cables and improving their sound - making it more fluid. That is quite easy to see - why should not there be a set of similar effects working on a whole loudspeaker?