how do i get my pc to work with my stereo?

[Twig]

i have connected my pc to my stereo and i can not get any sound i get all the default windows sounds come through the stereo but not if play videos or mp3s. i had my previous pc connected to my stereo and did not have this issue so can not understand why this setup does not work correctly with my new pc please help
the new pc is running on windows 7