Elastic constants of single crystals of metastable β-phase of the Ti-15Mo alloy were studied by ultrasonic methods with the aim to observe the dependence of these constants on formation of isothermal ω particles. Two ultrasonic methods were applied: resonant ultrasound spectroscopy for monitoring the temperature evolution of the elastic constants, and transient grating spectroscopy for identification of the local material symmetry at a fixed temperature. Samples with different heat treatments (isothermal ageing at 300 °C and ageing under the same temperature with uniaxial [111] loading) were studied. The results prove that the isothermal omega particles always exactly follow the original cubic symmetry of the β matrix, and that the evolutions of the elastic constants of the β-ω multi-phase crystals can be reliably approximated using Hill’s homogenizing procedure, assuming cubic elastic constants of the β-phase and isotropic elastic constants representing the ω particles.