Are you attempting to play sound through the 4670's codec (HDMI sound output) or through another sound source (motherboard's onboard sound or a dedicated soundcard)?
Go into your control panel and select 'Sounds and audio devices'.
Make sure which ever sound source you are trying to use is selected as default and that the volume is turned up.
Restart the application you are trying to use and see if there is sound.
If this does not help, please post your full system specs and some more details on your issue.
What kind of motherboard do you have? Many motherboards have a connection located close to the video card to feed motherboard audio into the audio channel of the video card. My Gigabyte motherboards call this the "SPDIF_O" header.
The 4670 has a build in Audio codec. It delivers sound through the HDMI port on the card or through a DVI to HDMI adapter. It installs another audio device in windows in addition to any onboard or expansion card audio you may have. Just make sure the one you are using is selected as the default device. I know when I plug an HDMI cable into my card it automatically assumes it wants to use audio from the card.