Usually an issue with having no sound in the sim  is related to the DirectX redistributable not being installed.

Navigate to:
C:\Run8Studios\Run8 Train Simulator V3\Redistributables\directx_Jun2010_redist

and run DXSETUP.exe. This should resolve the issue. Please note if you have your sim installed in a location other than the default please ensure you navigate to your Run8 Directory.