Check the basics first. Start>Control Panel>Sounds & Audio> on the Volume tab place a tick in the "place icon on task bar" (This gives quick access for later) On the audio tab check that the default devices look right for the machine. Click advanced on the sound playback section and check that nothing is set to mute. On the hardware tab check that there are no conflicts. Click Apply and OK. Try again.
Note that there is a troubleshoot button on the hardware tab. This can sometimes produce a result so if you`re still not up and running go back to it and use this wizard. If all this fails - post back!