KMix on Linux can use either the ALSA driver or the OSS driver, not both. If you really need to use both drivers at the same time (a very rare situation), you can do it as follows: Quit KMix and add the following line to your kmixrc file in the global configuration section.
MultiDriver=true
Start KMix again. If you click Help->Hardware Information you should see "Sound drivers used: ALSA0.9 + OSS" and "Experimental multiple-Driver mode activated"
Warning: You will probably see all of your mixers doubled.
There is no support for this kind of configuration.