  The temperature and high magnetic field dependence of the longitudinal
soundwave mode c_{11} in the two dimensional dimer system SrCu_{2}(BO_{3})_{2}
is presented. c_{11}(T) shows anomalies due to strong interdimer spin-strain
coupling. We can quantitatively interpret the temperature dependence of
c_{11}(T) together with the magnetic susceptibility $\chi_{m}(T)$ with a
molecular field approximation of coupled dimer triplets. The sound velocity up
to 50T shows very sharp softening between the magnetization plateaus at low
temperature. We argue that these pronounced effects arise from a resonant
interaction between the phonons and the magnetic excitations which show
softening between the plateaus.
