Answer
$v = \sqrt{\frac{Mg~sin(\theta)}{\mu}}$
Work Step by Step
The tension in the string is equal to the component of the block's weight directed down the slope. We can find the tension in the string.
$T = Mg~sin(\theta)$
We can find an expression for the speed of the waves on the string.
$v = \sqrt{\frac{T}{\mu}}$
$v = \sqrt{\frac{Mg~sin(\theta)}{\mu}}$
