itch.io

You can actually set the default volume in the code to make it easier. I found some code here; it's old but worked when I tested it. Paste the following at the start of the script.rpy file (make sure to preserve indents):

label splashscreen:
    python:
        if not persistent.set_volumes:
            persistent.set_volumes = True
        
            _preferences.volumes['music'] *= .45
            _preferences.volumes['sfx'] *= .67
return