This claim is quite similar to the idea that exposing a new guitar to vibrations from a speaker over a period of time can improve its tone. The underlying idea is that vibrations acting on the soundboard can alter the structure of the wood. This idea has always struck me as totally bogus. It may be that the structure of wood does change somewhat with age and with the loading exerted by the strings but the extra forces exerted by sound vibrations are likely to be too small to have any effect.
As far as I know such research that has been done does not support the idea that vibration induced changes affect tone. There is an online journal, the Savart Journal (published in collaboration with the Guild of American Luthiers) that features peer reviewed research articles and research notes on the science and technology of stringed musical instruments. There is an article from 2014 (CLEMENS, B., KADIS, J., CLEMENS, D., POLLAK, E., CLARK, P., GROVES, J.. Effect of Vibration Treatment on Guitar Tone: A Comparative Study. Savart Journal, North America, 1, Sept. 2014. Available at: <http://www.savartjournal.org/index.php/ ... le/view/22
>. Date accessed: 11 May. 2017.) entitled: Effect of Vibration Treatment on Guitar Tone: A Comparative Study
(admittedly carried out with steel-string guitars) that suggests this is not a noticeable effect. The abstract reads:
In order to study the widely-held belief that the sound of a guitar evolves with use due to vibration induced changes in the guitar, the tone of guitars subjected to controlled vibrations is investigated. The study uses three pairs of guitars, where each of the two guitars in the pair is the same make, model and year. One guitar from each pair is treated using a commercial device for effecting a tone change through imposition of vibrations. The guitars are evaluated before and after the treatment using double-blind player evaluations and physical property measurements. The player evaluations showed no statistically significant changes in the differences between the two guitars in each pair. Fourier analysis of instrumented hammer strikes were used to extract the frequency response function. Statistical analysis showed no significant change in the correlation between treated and untreated guitars due to the vibration treatment. It is therefore concluded that this vibration treatment had no significant effect on the guitar tone. It is suggested that the evaluation approach used here could be useful for studies of other instruments or treatments.