How to get velocity from PSD graph
You can get the sound pressure level from PSD, and the velocity is related to the sound pressure. According to the equation of motion,
$$
\rho \frac{\mathrm{d}\vec{v}}{\mathrm{d}t} = -\nabla p \tag{1}
$$
where $\rho$ is the density of the medium, $\vec{v}$ is the particle velocity, $\nabla=\frac{\partial}{\partial x}\vec{i}+\frac{\partial}{\partial y}\vec{j}+\frac{\partial}{\partial z}\vec{k}$ is the gradient operator, and $p$ is the sound pressure. When the sound is not so loud, the equation of motion can be linearized and the total derivative $\mathrm{d}\vec{v}/\mathrm{d}t$ becomes partial derivative $\partial{\vec{v}}/\partial{t}$.
Furthermore, if you make a plane wave assumption, then the particle velocity amplitude is
$$
v = \pm \frac{p}{\rho_0 c_0} \tag{2}
$$
for plane waves of forward and backward propagation, respectively. $\rho_0$ and $c_0$ are respective the medium density and sound speed without acoustic disturbance. Their product $\rho_0 c_0$ is the characteristic specific acoustic impedance of the medium.
However, your data is acquired in a cavity with an extremely high level of PSD, thus the linearization is obviously inappropriate. The total derivative in Eq. (1) should be
$$
\frac{\mathrm{d}\vec{v}}{\mathrm{d}t} = \frac{\partial \vec{v}}{\partial t} + \frac{\partial\vec{v}}{\partial x} \frac{\partial x}{\partial t}+ \frac{\partial\vec{v}}{\partial y} \frac{\partial y}{\partial t}+ \frac{\partial\vec{v}}{\partial z} \frac{\partial z}{\partial t} \\
= \frac{\partial \vec{v}}{\partial t} + v_x\frac{\partial\vec{v}}{\partial x}+ v_y\frac{\partial\vec{v}}{\partial y}+ v_z\frac{\partial\vec{v}}{\partial z} 
\tag{3}
$$
In addition, velocity is also related to frequency, but it seems that you want an overall velocity. You can obtain the time-domain sound pressure according to the Parseval's theorem, and then calculate the particle velocity.