The observer function will get an 'event' object passed in and for course modules, the course module id is $event->objectid and the module information is in the $event->other array
'other' => array(
'modulename' => $cm->modname,
'instanceid' => $cm->instance,
'name' => $cm->name,
)
so you can use that to get the table record e.g.
$record = $DB->get_record($event->other['modname'], array('id' => $event->other['instanceid']);
Cheers