well it's deffinetly there, cause i've seen it.. it's under the Main Bios... you will see a load of thing.. saying disable, enabled.. and somewhere down the list (big list) you will see On-Board Sound = just select to Disable.
RIG
Asus A7N8X Deluxe
80gb Maxtor
Lian-Li PC-60
Lite-On 52X
AMD XP2800+
LeadTek GeForce 4 Ti4200 w/vivo 128mb 8x
Hitachi CML174
1 GB Corsair XMS PC3200 Cas2