What sound card do you have?
Usually you will need to load a module. To search available modules look in the /lib/modules/2.4.18/kernel/drivers/sound directory.
Locate the one you need and then use "modprobe" to load the module. You can use the "lsmod" command to list the currently loaded modules.
If you have a really new sound card without kernel support then you may need to use the alsa drivers available at http://www.alsa-project.org.