having problems installing hardware after upgrade

I have Windows XP Pro and I'm having problems getting sounds to play on my computer. All the plugs are plugged into the right places, the power light is on, but when I go into the Control Panel, under Sounds and Audio devices, it just keeps telling me that there's no audio device. The speakers that came with the computer won't work and neither will my new set. When I try to bring up the Volume Control to see if maybe it's all muted, it tells me that "There are no active mixer devices available. To install mixer devices, go to Control Panel, click Printers and Other Hardware, and then click Add Hardware. This program will now close." ???? What do I do??