I think dimdim should increase the number of attendees to use the mike.
You can increase the number of attendee mikes by modifying the dimdim.properties file. The path is C:\Program Files\Dimdim\Conference Server\Tomcat 5.5\webapps\dimdim\WEB-INF\classes\resources
And the property to be modified is
This takes a maximum of upto 6 - so a meeting can have a total of 7 speakers.