At least we call it "settling" with guitars where I come from.  Maybe there is a different term used with violins or a more proper term for it, I don't know.

What I am talking about is when you get an instrument brand new (in the sense that it has not been played before or even kept tuned), or any major change is made like a repair or changing to a very different type of strings or a new bridge or something, it may sound "ok", but after a month or few of playing something happens.  You may notice it when you pick the instrument up or you may notice it while playing, but the instrument just suddenly sounds better.  Like it just figured out it can "sing".

According to a guitar repairman I've learned a good bit from over the years, it is a physical change that occurs because all the parts "shake down" and finally find a stable arrangement that works with the pitches/frequencies the instrument is being played at.  Though the instrument may have seemed stable before, some parts may have moved some tiny amount like a thousandth of an inch or something so they just all work together better.  I have heard some people claim it is a myth or even "superstition", or just "all in the player's head", but I have personally heard it happen a few times over the years.  It's part of why an older instrument that has been well cared for and was played often can be more desirable than a brand new one, if you have the option.  Some new instruments never seem to do it, or maybe they would take years to do it, and they are kind of the "lemons" among musical instruments.

It is usually a somewhat dramatic change, where the instrument may suddenly sound a bit louder and "sweeter".  Sometimes it will switch back and forth between settled and unsettled a few times before stabilizing with "the new sound".  This can be very odd if it happens when you are actually playing the instrument.

I had thought that my little electric violin probably wouldn't have any such change happen because other than the bridge, it is pretty much entirely made of man-made materials.  But last week it did.  I was practicing and it started sounding different.  Kind of like the difference you might hear if you touched the scroll of a violin to a door or wall while playing, that obvious of a change in volume/tone.  It may have been more noticeable, since being an electric the acoustic sound from it is pretty quiet anyway.  It shifted back and forth a few times (I wish I'd had the presence of mind to record the change as it was occurring) before it finished.  I still wouldn't say it sounds great when played acoustically, but at least it sounds better.

Anyway, I thought I would mention it since (if you choose to believe it is not a "myth") I have also been told that instruments can also be soured, if they are left out of tune.  It is why I always check the tuning before I put any instrument away or put it up for the rest of the day/night.  If the instrument is kept in tune, settling is said to occur to help the instrument sound "sweeter" when  it is in tune.  If the instrument was badly out of tune when it happens, the settling may make the instrument more inclined to non-musical pitches/frequencies and can result in it sounding less good than it can, no matter how carefully one tunes it.

From what I understand, it is usually most dramatic when the instrument is new or has not been played in a long time, but changing strings or the frequency standard one tunes to (like if one uses a lower historical pitch to tune to rather than the accepted modern A440) or changing a major piece in the vibrational path like the nut, bridge or tailpiece may also result in settling, but usually not as drastic as when it happens the first time.  It can be why someone may like the sound of a set of strings or a bridge or whatever when they hear it on another person's instrument but not like the sound when first trying them on one's own instrument.  It can take days, weeks or months for the instrument to fully re-settle after a change like that.

It can also be why instruments "straight from the factory" often sound worse than instruments from a good music shop where they were kept tuned up and may have at least partially settled before being bought.

Anyway, Oliver's post on the string company guy saying the strings would stretch into tune before the violin does reminded me of it, so I thought I would bring it up and see what folks here have to say about it.