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We investigate elastic, vibrational, and thermal properties of hexagonal (lonsdaleite) Si𝑥⁢Ge1−𝑥 alloys in the framework of density functional theory. We calculate elastic stiffness constants and phonon frequencies across the full composition range and analyze their dependence on chemical composition and atomic arrangement. While the elastic moduli show an almost linear composition dependence, phonon energies and dispersion are significantly affected by the specific arrangement of Si and Ge atoms on the hexagonal lattice sites. From the elastic properties, we determine the propagation of sound waves and the critical layer thickness for epitaxial growth. We further analyze global thermal quantities including heat capacities and thermal expansion coefficients as functions of composition and temperature, revealing negative thermal expansion at low temperatures. All calculated properties are compared with theoretical and experimental results for cubic Si𝑥⁢Ge1−𝑥 alloys, revealing remarkable quantitative similarities, attributed to the identical tetrahedral coordination of both structures.