While you can turn sounds on/off and assign other .wav files with the sound panel of the Options dialog, there isn't any "volume" adjustment.
You can, however, obtain the original set of the sound file here:
Just make a directory: \program files\clipmate7\sounds
Then unzip the sound file into that directory, edit as required with the windows sound recorder (start | programs | accessories | entertainment | sound recorder), and assign within ClipMate's sound dialog (Tools | Options | Sounds). Use the "Custom" option, then the "Browse" button to select the new sound file. Override each sound event, as required.
We have prepared a set of "quiet" sounds using this technique:.