What is hd audio codecs?
HD = High definition Audio = Sound, stuff you can hear or listen to Codec = System widely used in order to enjoy various media types. For example, a video might me in .AVI or .AMV or .MPEG format, a codec translates this coding (the AVI or whatever) so that you can enjoy said media.
Codecs enable you to play various formats in a media player.
Dolby Surround 5.1