Ac3 is a sound file format which is used on DVDs and Blu-ray discs.
For more details please see ac3 at Wikipedia.
It is used here to encode multichannel surround music. The files can be played for example with mediaplayers like VLC media player. The Ac3 filter can be used to play this kind of files with other media players.
The free lossless audio codec is used for high quality audio coding. It works with stereo as well as with multichannel surround sound. A codec can be downloaded at the project site.
Firefox had some issues in playing mp3 files (licencing). Since May 2013 Firefox is able to play mp3 via the HTML5 media player function. Possibly you need to set a switch in the configuration. To do so type about:config in the address line. Find the media.windows-media-foundation.enabled entry, set it to true and restart firefox.
If you want to enjoy the 5.1 surround files in the way they were meant to sound please make sure your sound system is correctly set to a 5+1 speaker configuration.Please find further information on playing surround music as well as some test files at Lynne Music. (By the way: there is also some great surround music!)