  We present CO(2-1) and $^{13}$CO(2-1) maps of the Cygnus-X molecular cloud
complex using the 10m Heinrich Hertz Submillimeter Telescope (SMT). The maps
cover the southern portion of the complex which is strongly impacted by the
feedback from the Cygnus OB2 association. Combining CO(1-0) and $^{13}$CO(1-0)
maps from the Nobeyama 45m Cygnus-X CO Survey, we carry out a multi-transition
molecular line analysis with RADEX and derive the volume density of
velocity-coherent gas components. We select those components with a column
density in the power-law tail part of the column density probability
distribution function (N-PDF) and assemble their volume density into a volume
density PDF ($\rho$-PDF). The $\rho$-PDF exhibits a power-law shape in the
range of 10$^{4.5}$ cm$^{-3}$ $\lesssim n_{\rm H_2} \lesssim$ 10$^{5.5}$
cm$^{-3}$ with a fitted slope of $\alpha = -1.12 \pm 0.05$. The slope is
shallower than what is predicted by simulations of rotationally supported
structures or those undergoing gravitational collapse. Applying the same
analysis to synthetic observations with feedback may help identify the cause of
the shallow slope. The $\rho$-PDF provides another useful benchmark for testing
models of molecular cloud formation and evolution.
