  We show that the volume of a simple Riemannian metric on $D^n$ is locally
monotone with respect to its boundary distance function. Namely if $g$ is a
simple metric on $D^n$ and $g'$ is sufficiently close to $g$ and induces
boundary distances greater or equal to those of $g$, then $vol(D^n,g')\ge
vol(D^n,g)$. Furthermore, the same holds for Finsler metrics and the
Holmes--Thompson definition of volume. As an application, we give a new proof
of the injectivity of the geodesic ray transform for a simple Finsler metric.
