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