  Molecular outflows are a direct consequence of accretion, and therefore they
represent one of the best tracers of accretion processes in the still poorly
understood early phases of high-mass star formation. Previous studies suggested
that the SiO abundance decreases with the evolution of a massive young stellar
object probably because of a decay of jet activity, as witnessed in low-mass
star-forming regions. We investigate the SiO excitation conditions and its
abundance in outflows from a sample of massive young stellar objects through
observations of the SiO(8-7) and CO(4-3) lines with the APEX telescope. Through
a non-LTE analysis, we find that the excitation conditions of SiO increase with
the velocity of the emitting gas. We also compute the SiO abundance through the
SiO and CO integrated intensities at high velocities. For the sources in our
sample we find no significant variation of the SiO abundance with evolution for
a bolometric luminosity-to-mass ratio of between 4 and 50 $L_\odot/M_\odot$. We
also find a weak increase of the SiO(8-7) luminosity with the bolometric
luminosity-to-mass ratio. We speculate that this might be explained with an
increase of density in the gas traced by SiO. We find that the densities
constrained by the SiO observations require the use of shock models that
include grain-grain processing. For the first time, such models are compared
and found to be compatible with SiO observations. A pre-shock density of
$10^5\, $cm$^{-3}$ is globally inferred from these comparisons. Shocks with a
velocity higher than 25 km s$^{-1}$ are invoked for the objects in our sample
where the SiO is observed with a corresponding velocity dispersion. Our
comparison of shock models with observations suggests that sputtering of
silicon-bearing material (corresponding to less than 10% of the total silicon
abundance) from the grain mantles is occurring.
