Do sound boxes of acoustic instruments (guitar,violin etc) get better with time (decade or so)?
That is their overall resonance is more pronounced. Or is it just placebo effect ?
Wood "lives". Certainly the sound of a wooden instrument can change over time. Most obvious, if the wood wasn't expertly selected and dried there's a high danger of deforming later, this will usually make the instrument worse overall. To some degree this can happen even in a good instrument if it's stored at extreme humidity or temperature conditions, again usually for the worse.
...except, there's not really an objective "good" or "bad" in musical instruments. Usually when a neck deforms under string tension it'll render the instrument unplayable (or at least badly), but sometimes the increased height gives the strings the space they needed to fully exploit the instrument's capabilities. I have a cheap-ish acoustic bass guitar with really low action, which I never was quite happy with. I deliberately placed it next to the conditioning machines I've had installed at my place after a piping damage, and to my amazement the action increased so the bass now sounds much better. Obviously I've had quite some luck here.
Even under ideal storage conditions, the instrument won't stay exactly as it was. If played regularly, there's some inevitable wear e.g. to the fingerboard. The vibrations (or lack thereof, when never played) affects the wood in ways that are, I think, not fully understood by anyone. Again, any change can be good or bad, it's pretty much impossible to tell beforehand.
Resonance is not a one-parameter thing. It doesn't simply "get more pronounced" – or less – when an instrument ages. Acoustic instruments vibrate in a multitude of modes, and all of these modes can change their center frequency, dampening, and nonlinearity amount. This can result in a benefitial boost of some frequency range, it can even out the response, it can cut away undesirable parts, but it can also cause nasty interference beating / wolf tones, loose the original character, or start to produce "dirty" nonlinear distortions.
Yes. But not just time, its "playing hours tuned" that makes the difference.
Wood is a soft material that adjusts with time, humidity etc. If you have a new instrument and play on it with good tuning (keeping same reference A, for example A = 440Hz) the sound quality and overtone richness will increase. You will get a more complete sound out of the instrument.
Many luthiers put brand new violins "lystening to music". They put speakers around the instrument and play music to it in the same reference A tuning to make the instrument vibrate and thus the whood adapt to certain vibrations.
If you get a violin that has not been played for a long time you will notice the huge difference in sound after playing on it 1 week some hours everyday.
I answer from my violinist perpective but assume the process is similar in other wood string instruments.
I once read about a project that scanned an antique Stradivarius in an MRI machine; a luthier used the resulting scan to produce the exact same violin out of new wood. In a blind test no preference was shown. Of course, the quality of strings, and in case of a violin, the bow, all add to the quality of the sound, along with the playing technique and hall. As a pianist, I can usually tell the difference between the sound quality of a good or bad piano, but rarely an old or new one.