After i got my Apple TV 2 jailbroken and installed XBMC on it I started to try and play some mkv files on it. I quickly discovered that it was pure misery. The movie wouldn’t buffer like they should and some wouldn’t even play. I startet googling and discovered a lot of people with the same problems as me. To get it fixed you have to install XBMC Nightly Builds. It has something to do with the lastes version of iOS. After it’s done the files will play like they should. You can see how to install it below.

1. Log in to your Apple TV through ssh: ssh root@<your ATV ip address here> from terminal.app (or similar) and enter your password (if it’s the first time you log on through ssh the password is alpine - you can also change this with the command passwd).

2. Add the missing repository with this command: 
echo “deb http://apt.saurik.com/ tangelo-3.7 main”>/etc/apt/sources.list.d/saurik.list
Then do a: apt-get update and after that: apt-get -f install

3. Get the Nightly Builds install files:
wget http://hadm.net/~keith/xbmc_atv2/latest_atv2.deb (hit enter and wait for it to download). When it’s down do a: dpkg -i latest_atv2.deb (and wait for it to finish). Then remove the installer file with: rm latest_atv2.deb

4. To make sure that all dependencies has been installed do a: apt-get -f install

5. Finish of with the command: killall AppleTV

Congrats, your now running XBMC Nightly Builds.