$$v = {1 \over {\sqrt {{\varepsilon _m}{\mu _m}} }}$$

$$v = {1 \over {\sqrt {{\varepsilon _0}{\varepsilon _r}{\mu _0}{\mu _r}} }}$$

Since $$c = {1 \over {\sqrt {{\varepsilon _0}{\mu _0}} }}$$

$$ \Rightarrow v = {c \over {\sqrt {{\varepsilon _r}{\mu _r}} }}$$