  The freezing and melting of $^{4}$He in various aerogels with porosities
ranging from 92 to 97% was studied using longitudinal ultrasound. The freezing
pressure, detected by a change in velocity and attenuation, was elevated from
that of bulk by about 0.3 MPa, and showed little dependence on aerogel
porosity. We report measurements of the transmission of sound through the
solid-liquid interface in aerogel, for the first time, which was independent of
temperature and unaffected by a small addition of $^3$He, unlike the bulk
interface. This indicates that crystal growth was suppressed by disorder,
originating from silica strands. However, small amounts of $^3$He in the solid
significantly decreased the attenuation, because the resultant pinning of
dislocations by $^3$He suppressed phonon scattering, as is observed in bulk
solid $^4$He.
