What causes an increase in sound speed in a medium? Its an established fact that increase in the temperature causes increase in speed of sound waves but what is the property which is changed by changing temperature ? Does frequency and wavelength get affected by temperature?
 A: The speed of sound is given by the Newton-Laplace equation:
$$ v = \sqrt{\frac{K}{\rho}} $$
where $K$ is the bulk modulus (i.e. a measure of stiffness) and $\rho$ is the density. The physical interpretation of this is fairly obvious. Stiffer substances recoil faster from a displacement so increasing the stiffness increases the speed of sound. Heavier substances recoil more slowly from a displacement so increasing the density decreases the speed of sound.
The effect of temperature lies in how it changes $K$ and $\rho$, but the effect will vary for different materials. For an ideal gas the the bulk modulus P is simply the gas pressure multiplied by the adiabatic index, $\gamma$, so the speed is given by:
$$ v = \sqrt{\frac{\gamma P}{\rho}} \tag{1} $$
We can manipulate this equation using the ideal gas formula:
$$ PV = nRT $$
For example the density is $nM/V$, where $M$ is the molar mass of the gas, so:
$$ \rho = \frac{nM}{V} = \frac{PM}{RT} $$
If we make this substitution in equation (1) we get:
$$ v = \sqrt{\frac{\gamma RT}{M}} $$
giving us the result that the speed of sound increases with temperature as you said in your question.
