The mplayer package for Maverick is listed in Packages but it's not present in the repository, hence installation fails:
th@eee:~$ sudo apt-get update
Hit http://archive.canonical.com maverick Release.gpg
...
...
Hit http://packages.medibuntu.org maverick Release.gpg
Ign http://packages.medibuntu.org/ maverick/free Translation-en
Get:3 http://dl.google.com stable Release [2,544B]
Ign http://gb.archive.ubuntu.com/ubuntu/ maverick/multiverse Translation-en
Get:4 http://gb.archive.ubuntu.com/ubuntu/ maverick/multiverse Translation-en_GB [91.2kB]
Ign http://packages.medibuntu.org/ maverick/free Translation-en_GB
Ign http://packages.medibuntu.org/ maverick/non-free Translation-en
Ign http://packages.medibuntu.org/ maverick/non-free Translation-en_GB
Hit http://packages.medibuntu.org maverick Release
Ign http://gb.archive.ubuntu.com/ubuntu/ maverick/restricted Translation-en
...
...
...
Fetched 224kB in 1s (220kB/s)
Reading package lists... Done
th@eee:~$ sudo apt-get install mplath@eee:~$ sudo apt-get install mplayer
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
libfaac0
Suggested packages:
mplayer-doc netselect fping
The following NEW packages will be installed
libfaac0
The following packages will be upgraded:
mplayer
1 upgraded, 1 newly installed, 0 to remove and 1 not upgraded.
Need to get 2,902kB/2,964kB of archives.
After this operation, 160kB of additional disk space will be used.
Do you want to continue [Y/n]?
Err http://packages.medibuntu.org/ maverick/non-free mplayer i386 2:1.0~rc4~try1.dsfg1-1ubuntu1+medibuntu1
404 Not Found
Failed to fetch http://packages.medibuntu.org/pool/non-free/m/mplayer/mplayer_1.0~rc4~try1.dsfg1-1ubuntu1+medibuntu1_i386.deb 404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
Obligatory "me too" post. mencoder is also missing.