Maybe try Linux Mint? this is a downstream distro from Ubuntu, which among other things includes many built in media codecs. Using it I've never had a codec issue, so I don't know if this is what they look like. Also, on mintcast.org they have the first part of a walkthrough they are doing on setting up mythTV for a HTPC.
The reason that Ubuntu does not include the codecs by default is that they are closed source and not something that they maintain. If you want them they are but a few mouse clicks away. For a HTPC build I would think that Mythbuntu would be ideal as Myth is set-up from the start so near zero configuration required.
In terms of the sound issue is this still a problem for the OP?