It was pointed out to me last week that Google Docs has changed the file format for Presentations from ppt to pptx. When an assignment is submitted to Moodle, the file extension is ppt. Powerpoint will throw an error unless the extension is changed to pptx. I have hacked the /lib/googleapi.php file on my installation from ppt to pptx, and it works just fine.
Please see MDL-32223.