"HD Audio" is the type of audio chip used by the G4. Its not meant to refer to some new standard audio format like Dolby Digital. It doesn't say what each channel is, whether its left, right, center, etc. It just says the hardware has support for audio formats that use up to 8 channels. When you are playing a DVD with dolby digital sound, it will use 6 of these channels to create the 5.1 audio. This is the most you'll usually ever encounter. I wouldn't even know where to get media that supports more then 6 channels.
The G4 has 6 outputs. FR, FL, RR, RL, Center, and Sub. Which is enough to fulfill the needs of every type of media file generally available. The FR, FL, RR, and RL outputs are speaker level outputs. If you need an optical output or want to run the LR speaker audio through an amp, you can use an external sound card of which there are many types to fulfill probably any need you can think of. The turtle beach sound card sold as an option for the G4 will work for this. FM Radio currently doesn't work on the G4 with an external sound card, but there should be a fix very soon that will address this issue before we start shipping the G4s again.
There is really no such thing as a 8.1 audio standard though. If you are getting higher then 6.1 audio, the additional channels are more then likely artificially generated from a source with lesser channels. CD Audio, FM radio, most MP3s / music files, XM Radio, and, in general, most other audio sources are just 2 channels. When you have just a stereo source, to get output on a 6.1 system, you are normally just copying the front speaker sound in some way to the other audio channels. DVDs are normally 5.1 channel and the G4 as well as the T3 supports this since they have 6 different outputs. If you are hooking up more then 6 speakers to your system, you can just split the channels that you need and send them to additional speakers.