Also, when using the 'root' (Super-Admin) account, you need to use 'root' as your username, NOT your normal user id. This is because root is a completely different and seperate account.
My thought is that maby you are trying to use your normal account's name. Use 'root' instead, along with the root user's password. You shouldn't have any problems with authentication.
Also, which Ubuntu install are you using?
There is actually a specific Ubuntu install for video, sound, and graphical editing tools.. it has some really powerful tools... pretty much, the best free opensource tools around for editing.
Home | Ubuntu Studio <---The version I speak of.
If you haven't installed this version, you can still get the other opensource software.. you'll just have a harder time finding, configuring, installing.... etc.
(I can't remember if Ubuntu uses package installs like Debian or Debian deritatives, *cough*Knoppix*cough*...) If it does have package installs, and doesn't require sourcecode compilation, you should just be able to browse an index and find the installs.. It'll be somewhere in the 'system' options, if it is there. I just don't remember! xD
~~Azurith