Ya I know I'm the guy who does Ultumix but this is a problem for both of us.
ENVY depends on Ubuntu's repository to get the NVIDIA and ATI driver and Ubuntu has dumped their repo for ENVY. Someone needs to make an update so that ENVY will work with a new repo.
Also for users who want to install say an NVIDIA driver manually (welcome back to the dark ages) you need to go to http://www.nvidia.com and download your driver for your video card and save it to the Desktop. Then in (lets say Ultumix) you will need to reboot and go into the recovery mode and log in as root in the terminal and cd /home/owner/Desktop (replace owner with your user name) then type sh ___________.run (________ represents the name of the driver you downloaded) If you don't know what ________ is just type dir and press enter and you should see it. REMEMBER it's sh and not su.