Having the same problem as 'Sound Advice' in that
I keep getting 'Couldn't open Mixer Device /dev/sound/Mixer'
when I open up X windows (Gnome). I have tried all
the suggested fixes:

chmod 777 /dev/dsp
chmod 777 /dev/mixer

as well as;

chmod 777 /dev/audio
chmod 777 /dev/sequencer

Still getting the same message.

Motherboard employs VIA VT8233 AC97 Audio Controller.
Support for this device is compiled into the kernel (2.4.20)
I am trying to use the onboard sound, not a separate
card.

What am I missing?

If you problem is when you are logged in as just a normal user, then you should use the commands listed with an "*" on the end to change the permissions of all the devices in the category. For example: chmod 777 /dev/dsp*

I do not know why a program would be attempting to open a device in a subdirectory that probably doesn't even exist. At least I know I do not have a /dev/sound directory. If you are unable to change the reference to the normal /dev/mixer device within the program being attempted, then I guess you should make a link for it as a work around.

mkdir /dev/sound
ln -s /dev/mixer /dev/sound/Mixer

Perhaps it will help.

Unfortunately the solution to this problem was much more
complicated than I expected. Reading in another forum, I
came across a reference to the VIA 82c686. Supposedly
this hardware requires the use of ALSA drivers.

Apparently, most distributions don't carry support for much
of the sound cards and on-board sound hardware that is
found on many machines. ALSA (Advanced Linux Sound
Architecture) is the answer. Fortunately, ALSA drivers and
lib/utilties software was available for the VT8233 at the
ALSA site:

http://www.alsa-project.org/

Scanning the sound card matrix produced a page with install
instructions for the latest release - 0.9.1. After following the
./configure;make;make install instructions for the driver, libs
and utilities, sound is now flowing nicely.