  Ultraluminous X-ray sources (ULXs) with luminosities lying between ~3x10^{39}
- 2x10^{40} erg/s represent a contentious sample of objects as their
brightness, together with a lack of unambiguous mass estimates for the vast
majority of the central objects, leads to a degenerate scenario where the
accretor could be a stellar remnant (black hole or neutron star) or
intermediate mass black hole (IMBH). Recent, high-quality observations imply
that the presence of IMBHs in the majority of these objects is unlikely unless
the accretion flow somehow deviates strongly from expectation based on objects
with known masses. On the other-hand, physically motivated models for
super-critical inflows can re-create the observed X-ray spectra and their
evolution, although have been lacking a robust explanation for their
variability properties. In this paper we include the effect of a partially
inhomogeneous wind that imprints variability onto the X-ray emission via two
distinct methods. The model is heavily dependent on both inclination to the
line-of-sight and mass accretion rate, resulting in a series of qualitative and
semi-quantitative predictions. We study the time-averaged spectra and
variability of a sample of well-observed ULXs, finding that the source
behaviours can be explained by our model in both individual cases as well as
across the entire sample, specifically in the trend of hardness-variability
power. We present the covariance spectra for these sources for the first time,
which shed light on the correlated variability and issues associated with
modelling broad ULX spectra.
