The influence of defects on different physical properties of crystals is one of the important questions in the solid state physics. Controllable changes in the intrinsic activity of deep level defects and vacancies are one of the key goals in the fabrication of new materials. Here we demonstrate the possibility of changing of the mechanical properties of the low - dimensional TlInS2 layered – semiconductor by engineering defects using a simple annealing technique.  TlInS2 is ferroelectric – semiconductor crystals having an incommensurate (INC) phase. It has centrosymmetric monoclinic space group at room temperature. On cooling from ambient temperature, TlInS2 undergoes successive phase transitions: from paraelectric phase to INC – phase at Ti ~ 216 K and from the INC - phase to polar ferroelectric commensurate phase at Tc ~ 200 K.  Ferroelectrics with INC - phase are well known to exhibit pronounced memory effect. The memory effect is caused by the formation of the long - live metastable states, related with the defects density wave (DDW). DDWh is formed in the crystal, as a result of the spatial redistribution of impurities and structural defects in the periodic field of the INC – phase. The effect has been attributed to the coupling of mobile defects to the modulation order parameter, forcing the defects to diffuse in the INC - modulated potential [1].  The influence of DDW on the longitudinal and transverse ultrasound wave propagation velocities at different temperatures has been investigated experimentally and theoretically. The acoustic properties of given compound were studied by the pulse - echo method. It is shown that elastic constants C33 and C44 of TlInS2 are drastically changed after annealing of the crystal within the INC - phase. It was shown that DDW may form the new defect energy state in phonon spectra of material which can be easily identified from ultrasonic measurements. The material remembers this defect energy state induced by memory effect after temperature treatment of TlInS2 within INC – phase. The changings in temperature behavior of C33 and C44 elastic constants of TlInS2 material on DDW - modulated structure corresponding to the memory effect have been established and explained.  Ab initio calculations of the full phonon dispersion and vibrational density of states were performed by ABINIT program [2] for TlInS2. The phonon band structure was calculated using density functional perturbation theory in the linear response approach [3], in which the second order derivatives of the total energy were calculated to obtain dynamical matrix. Phonon dispersion curves in the present calculation of phonon properties were performed using LDA exchange correlation functional.