  In this work, we study magnetoelastic interactions by means of ultrasound
experiments in $\alpha$-RuCl$_3$ -- a prototypical material for the Kitaev spin
model on the honeycomb lattice, with a possible spin-liquid state featuring
Majorana fermions and $\mathbb{Z}_{2}$-flux excitations. We present results of
the temperature and in-plane magnetic-field dependence of the sound velocity
and sound attenuation for several longitudinal and transverse phonon modes
propagating along high-symmetry crystallographic directions. A comprehensive
data analysis above the ordered state provides strong evidence of phonon
scattering by Majorana fermions. This scattering depends sensitively on the
value of the phonon velocities relative to the characteristic velocity of the
low-energy fermionic excitations describing the spin dynamics of the underlying
Kitaev magnet. Moreover, our data displays a distinct reduction of anisotropy
of the sound attenuation, consistent with randomization, generated by thermally
excited $\mathbb{Z}_2$ visons. We demonstrate the potential of phonon dynamics
as a promising probe for uncovering fractionalized excitations in
$\alpha$-RuCl$_3$ and provide new insights into the $H$-$T$ phase diagram of
this material.
