  Elastic properties of the filled skutterudite compound SmOs$_4$Sb$_{12}$ have
been investigated by ultrasonic measurements. The elastic constant
$C_{11}(\omega)$ shows two ultrasonic dispersions at $\sim$15 K and $\sim$53 K
for frequencies $\omega$ between 33 and 316 MHz, which follow a Debye-type
formula with Arrhenius-type temperature-dependent relaxation times, and remain
unchanged even with applied magnetic fields up to 10 T. The corresponding
activation energies were estimated to be $E_2$ = 105 K and $E_1$ = 409 K,
respectively. The latter, $E_1$, is the highest value reported so far in the
Sb-based filled skutterudites. The presence of magnetically robust ultrasonic
dispersions in SmOs$_4$Sb$_{12}$ implies a possibility that an emergence of a
magnetically insensitive heavy fermion state in this system is associated with
a novel local charge degree of freedom which causes the ultrasonic dispersion.
