You have created the device files on the disk (/dev/dsp and /dev/mixer). When you try to access these files, the kernel will map the file I/O requests on to the appropriate device driver (kernel module) to handle it.
The problem here is that the device driver can't be found. What you need to do is load the appropriate module to handle these devices, which depends on your sound card.
is likely to be a good start, but you may need to go through your /lib/modules/ directory and pick out the appropriate module from the list.
If you have problems when loading the modules, then you will need to know about
which lists the modules that you have loaded (q to exit), and the file
which is where error messages get sent.