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Phonon densities of states (DOS) were obtained from inelastic neutron scattering measurements on Ce 0.9 Th 0.1 at temperatures from 10 to 300 K. The α phase showed a significant softening of its phonon DOS when heated from 10 to 140 K. Despite the 17% volume collapse, the phonon DOS showed little change between the γ phase at 150 K and the α phase at 140 K. This is supported by analysis of the magnetic spectra showing that most of the transition entropy can be accounted for with the crystal field and changes in the ground-state spin fluctuations. We argue that the anomalous behavior of the phonon DOS originates with the volume dependence of the ground-state spin fluctuations.