Hrm. The menu at the bottom of the screen has a slider for the volume...that's not the problem, is it? Otherwise I'm not sure what happened, assuming you haven't moved any files out of the main Machinarium folder. (Meaning I know that the App Store will automatically install the Machinarium folder in your Applications folder, so have you moved any files contained inside of the Machinarium folder to another folder on you hard drive?)
I'm just a fan and not one of the developers, but you can contact the developers and let them know about the issue. You can use the form at the bottom of this page:
http://amanita-design.net/contact.html(And please post back to follow up once you've received a reply from them.)
