We numerically calculate the forms and frequencies of mechanical whispering-gallery modes in silica shells. Such modes were recently experimentally observed in water-filled optomechanical resonators, which constitute a bridge between optomechanics and microfluidics. We consider the three acoustic mode families of Rayleigh-Lamb waves, longitudinal waves and Love waves. Our study shows that these acoustic modes have rich velocity dispersion characteristics and can create considerable deformation of the inner surface of the shells. In this manner, a novel optomechanical interaction may be facilitated between fluids and light.