Cavity optomechanical systems enable coherent photon-phonon interactions essential for quantum technologies, yet high-performance devices have been limited to suspended structures. Here, we overcome this limitation by demonstrating cavity Brillouin optomechanics in a suspension-free racetrack microring resonator on a lithium-niobate-on-sapphire chip, a platform that merits high stability and scalability. We demonstrate coherent coupling between telecom-band optical modes and a 9.6 GHz phonon mode, achieving a maximum cooperativity of 0.41 and a phonon quality-factor-frequency product of ${10}^{13}\mathrm{Hz}$. The momentum-matching condition inherent to traveling-wave Brillouin interactions establishes a one-to-one mapping between the optical wavelength and phonon frequency, enabling multichannel parallel operations across nearly $300\mathrm{MHz}$ in phonon frequency and $40\mathrm{nm}$ in optical wavelength. Our suspension-free architecture provides a coherent photon-phonon interface compatible with wafer-scale integration, opening pathways toward hybrid quantum circuits that unite photonic, phononic, and superconducting components on a single chip.

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• Brillouin scattering & spectroscopy
• Integrated optics
• Nonlinear waveguides
• Optical microcavities
• Optomechanics