A modified definition of root-mean-square speed of molecules is shown to lead to a modified form of Newton formula for isothermal speed of sound in ideal gas. The calculations based on modified theory and those based on classical adiabatic theory of Laplace are compared with available experimental data for (Air, N2, O2, Ar, He, CO2). Also, a modified formulation for propagation speed of long waves in canals by Chandrasekhar is presented leading to formula identical to that of Laplace