In this study, changes in the vibro-mechanical properties of fungi-treated wood, during sorption and desorption at different humidity levels, were investigated. Norway spruce resonance wood (with uniform narrow annual rings and high tonal quality for musical instrument craftsmanship) was incubated with Physisporinus vitreus for 36 weeks. Stiffness, internal friction, and tonal performance indices of control (untreated) and fungi-treated wood were compared after exposure to a stepwise variation of relative humidity. It was demonstrated that fungal treatment increased the internal friction and decreased the specific modulus of elasticity, during reduction of wood density. Internal friction of both control and fungi-treated wood significantly increased during dynamic sorption, especially during early stages (hours) of each humidity change step. Both specific modulus of elasticity and internal friction showed a hysteretic behavior during humidity variation cycles. Hysteresis was smaller in fungi-treated wood. Also, tonal performance indices were improved after fungal treatment and showed a reduced variation at different relative humidity conditions. Dynamic vapor sorption tests and FT-IR microscopy studies revealed changes in hygroscopicity and the supramolecular structure of wood, which may explain the observed vibrational behavior. Less dependency of wood vibrational properties to the variation of the ambient humidity is important for the acoustic performance of string instruments.