  We discuss mixed inflaton and spectator field models where both the fields
are responsible for the observed density fluctuations. We use the current CMB
data to constrain both the general mixed model as well as some specific
representative scenarios, and collate the results with the model predictions
for the CMB spectral $\mu$ distortion. We find the posterior distribution of
$\mu$ using MCMC chains and demonstrate that the standard single-field inflaton
model typically predicts $\mu \sim 10^{-8}$ with a relatively narrow
distribution, whereas for the mixed models, the distribution turns out to be
much broader, and $\mu$ could be larger by almost an order of magnitude. Hence
future experiments of $\mu$ distortion could provide a tool for the critical
testing of the mixed source models of the primordial perturbation.
