Look in the [sound] section of your cp.ini file for the
wavetabledevices=devwMixF devwMixQ (...) devwMix devwNone
The leftmost device is used as default. So change the line to the following to enable the Normal Mixer:
wavetabledevices=devwMix devwMixF (...) devwNone
If you don't understand all this right now, read chapter 5 on how to configure OPENCP.