I don't know the right registry settings. There must be some a huge number of such keys since you can set up volume for different
sources and lines.
Probably somewhere under [HKLM\SYSTEM\ControlSet001\Control\DeviceClasses] (search for subkey mixer there).
However, it may be easier for you to pre-set master volume or wave volume with API call. You can use: mixerSetControlDetails(...,
MIXER_SETCONTROLDETAILSF_VALUE).
Good example of code that uses API you will find here: http://www.codeproject.com/audio/mixerSetControlDetails.asp