You said youtube so you are talking about online videos - and that
usually requires flash.

Click Applications > Add/Remove type Flash in the Search bar and
Macromedia Flash Plugin should come up at the top.

That fixed the sound, but the video is very jerky, as if it is skipping frames.
I still have no video on websites such as CNN.

Did you install the restricted extras for Ubuntu? These are proprietary
codecs and drivers that are not included with the distribution, but are in
the repositories.

You can install them from Synaptic, Add and Remove or from the command line.
In Synaptic search for restricted-extras and choose ubuntu-restricted-extras
or kubuntu-restricted-extras, etc.
In a terminal you can also use: sudo apt-get install
ubuntu-restricted-extras

They give you flash, windows ttfs and multimedia codecs such as libdvdcss2
which allows you to play movies.