to get libdvdcss2 i think you'll need to add deb
ftp://ftp.nerim.net/debian-marillat/ unstable main to your /etc/apt/source.list file. But if the problem is the sound do u use alsa or oss?
can you send the output of cat /proc/asound/version to see?
about apt-get it's very easy... for now you just need to use these commands (as root)
#apt-get update ---> well, update
#apt-get upgrade --> upgrade
#apt-get installl packagename --> install the package 'packagename'
for instance, if you don't have alsa you may want install it:
#apt-get update
#apt-get install alsa alsa-oss
Good luck!
A.