Check the following points:
- Multimedia plugin filter should be enabled under Site Administration > Plugins > Filter > Manage Filters > Multimedia plugins.
- Then it should be enabled in course and activity as well.
- The order of the filter under the Site Administration > Plugins > Filters > Manage filters should be correct.
To check the example refer the link http://moodle.org/mod/forum/discuss.php?d=194684