Install nforce drivers from nVidia:
http://www.nvidia.com/object/linux.html
This will install nvnet (onboard ethernet driver) and nvaudio (onboard sound driver). nvnet will solve your problem, but I would recommend going for ALSA sound drivers instead of nvidia's -- believe me, I also have a nForce-based mobo and ALSA drivers work much better, specially with Flash plugin.
HTH