The snapshots have all current changes/updates as of the moment they are created. If you have installed from a recent snapshop then you would have the recent changes already.
Many of the snapshots have been problematic with updates. If your snapshot install was prior to Aug 18, 2010 then there are some things you can do that should enable updates again properly. I'll try to make this easy and painless. Execute each of these from the terminal prompt as you have been doing already:
Remove the avenard repository from /etc/apt/sources.list:
sudo sed -e '/avenard/d' -i /etc/apt/sources.list
Re-order the local repository properly in sources.list:
sudo sed -e '/deb-cache/d' -e '1ideb file:\/usr\/pluto\/deb-cache .\/' -i /etc/apt/sources.list
Set apt preferences properly:
sudo echo 'Package: mythtv mythtv-frontend mythtv-backend mythtv-common mythtv-database mythtv-transcode-utils mythweb libmythtv-perl libmyth python-myth mythtv-themes mythplugins mytharchive mytharchive-data mythbrowser mythgallery mythgame mythmovies mythmusic mythnetvison mythnetvision-data mythnews mythvideo mythweather mythzoneminder myth-doc
Pin: origin deb.linuxmce.org
' | sudo tee /etc/apt/preferences
Update your package lists and try to upgrade:
sudo apt-get update
sudo apt-get upgrade
Try that out and let me know how it goes.