Install XiphQT

XiphQT Allows the playback of Ogg Vorbis (.ogg) .ogm and FLAC files.

Download

Download the latest decoder. (Make sure you download the .dmg marked 'only decoders'!)

Copy the .dmg disc image to your Apple TV

scp xiph-qt-decoders-0.1.8.dmg frontrow@appletv:

Login to your Apple TV via ssh. Mount the disc image

sudo hdiutil mount xiph-qt-decoders-0.1.8.dmg

The disc image will now be mounted in your /Volumes directory. eg: /Volumes/Xiph 0.1.8/

Make the file system read-write

sudo mount -uw /

Copy the Xiph components to your /Library/Components/ directory:

sudo cp -r "/Volumes/XiphQT 0.1.8/XiphQT (decoders).component" /Library/Components/

Make the file system read-only

sudo mount -ur /

Reboot your AppleTV (Is this necessary?)

sudo reboot

Update 03-Jan-2009 - After following the above steps on a 2.1 AppleTV, I had to then perform the following to get FLAC files to play in ATVFiles.

Download flac_import_0.5b1_p0.1.dmg from http://people.xiph.org/~arek/flac_import/ and transfer it over to your AppleTV using the same method as for the XiphQT dmg.

Mount it:

 sudo hdiutil mount flac_import_0.5b1_p0.1.dmg

Change into it's mounted location:

 cd /Volumes/FLAC\ Importer\ \(0.5b1\ +\ XiphQT\ patch\ 0.1\)/

Copy the contained component to your AppleTV's /Library/Quicktime directory:

 sudo cp -r FLACImport.component /Library/QuickTime/

Then restart finder or reboot your AppleTV:

 killall Finder

If you still have troubles playing your FLAC files, you might need to look at this thread for more details, check out this thread: http://forum.awkwardtv.org/viewtopic.php?f=11&t=1315