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 as:
$T = Mg~sin(\theta)$
We can find an expression for the speed of the waves on the string as:
$v = \sqrt{\frac{T}{\mu}}$
$v = \sqrt{\frac{Mg~sin(\theta)}{\mu}}$
