With HD Audio, there is a clear and audible difference is sound quality even on a 5.1 system. If you look at a Blu-Ray disc, a major part of the disc is used to store the audio. A Blu-Ray carries upto 8 channels of uncompressed audio, and this is mandatory. In addition, it can carry either Dolby TrueHD or DTS_HD Master audio, both of which are lossless encoding, again for upto 8 channels.
Bulk of the sound in any 5.1/7.1 encoding is used in the front three channels. When you use a HD Audio output, the dialogues are clearer, the sound of instruments and others are well separated, and it really has the capability of taking you right into the movie. Believe me, in movies with HD Audio, I have heard sound that I never knew existed in the movie.
One scene I quote very often is the war between the Romans and the Germanic barbarians in The Gladiator. When you play a DVD, you need a really good system to hear the panting of the dog as it races up the hill in front of the Roman horses. In a Blu-Ray, the sound is crystal clear. I can quote very many such scenes. The creak of metal, the chattering of cricket, the slam of a door, the creak of wood under a pair of shoes - everything is clearer and more realistic.
Cheers
Cheers