Make sure the volume isn't muted and is up.
Also, if ther'es a jumper for front panel audio, it may need to moved/removed.
i have installed windows xp and all the latest drivers. tried using realtek drivers still nothing. it will reconise that i have a microphone plugged in. but will not play sound
i have tried different headphones / speakers.
and it is set to auto in the bios!
Also make sure it's enabled in the BIOS. There is an "AC97 Onboard" setting under integrated peripherals.