It's because the HD standard also applies to sound, not just the picture:

"The third factor connected to the new HDTV-format is Sound Quality. In fact, most HDTV programming will carry with it Dolby Digital 5.1 surround sound, as frequently heard on DVD’s. So long as you have a surround sound unit attached to your television set, the surround sound will enable to the television viewer to be immersed in the sound, so much so as it often feels as if you are in the middle of the action happening on your television set." from MuchMorCanada Magazine.
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You may wish to include information such as manufacturer, model, and audio connection cables for different answers. I found very little in searching.
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