The software-mixer has its own file to save its state.
Have a look at <mocdir>/softmixer (e.g. /home/hiben/.moc/softmixer).
It should look something like this:

Active: 1
Amplification: 100
Value: 90
Mono: 0

The 'Value'-key represents the volume (0-100).
You can edit this file by hand but when moc exits, the file gets overriden (unless you set the 'Softmixer_SaveState'-option to 'no'.

Changing the file while moc is running has no effect. It is currently only possible to set the softmixer volume from the interface.

I hope this helps.