Are you aware that the MUC is configured by a special config file that lives in your moodledata area. The path is...
$CFG->dataroot . '/muc/config.php'
I don't have any particular insight into the contents but it looks (at a glance) reasonably intuitive.
By the way, if you mess up the MUC configuration, you can just delete this file and it will be recreated with default settings