Is there a reason you haven't updated to Ubuntu 10.04 or 10.10 yet on this
computer?

Check to see if your audio control is just muted. I had a similar problem and
had to go to audio control and unmute each time. I never resolved this
irritating default setting but I had to do other things.

On the top panel, toward the right, you should have a speaker icon. When you
click on it, one of the options might be "Sound Preferences."

I *think* nVidia MCP61 describes a motherboard, and your sound adapter is
probably a Realtek ALC888. If you open Accessories/Terminal and enter the
command: lspci
it will list many of your devices, and one of them is probably your sound
adapter.