multimedia audio controller windows xp dell dimension 8400 showing yellow question mark and exclamation. No sound. Went to dell ran diagnostics it did not see the hardware. I am using the onboard sound and the diagnostics don't see the hardware.
In your Device Manager (where you found the yellow exclamation), right click on the audio controller and select "uninstall". Then restart your computer, the computer should automatically detect the hardware and install drivers on restart. If the computer does not automatically detect the hardware, there may be a hardware issue but, you can attempt to "force" the driver by downloading it from Dell here http://www.dell.com/support/drivers/us/en/19/Product/di... and installing it manually - if there is a hardware issue that prevents the sound from working, likely the driver will fail installation with a message of "hardware not found".
Hope it helps