After installing Windows Vista on a XP computer my front audio port stopped working. I have a headphone that use on them. There was proper sound when I was using Windows XP but due to some issue the headphones are not working. I had connected a speaker on the backside which works well. So there is no issue of drivers I think. I checked in Device Manager. There is no exclamation mark on any hardware and it looks a proper system to me.
Check your sound card. Even if appropriate drivers are installed there might be issue with some internal hardware.The front audio port can have some hardware issue also. Click on the volume icon in the system tray and verify that you had selected the right mixer. You can go in Control Panel > Audio properties and look out for the setting. Sometime due to multiple codecs, or audio mixer the sound is affected. Try to check your pin connection also.
I faced a similar issue. I tried a number of things to fix the problem. But it looks Vista has some major issue with old drivers. I then purchased a sound card and added that to my system. After putting the sound card my problem was solved. Sound cards are quiet cheaper and you can buy the same from anywhere.