For the HD-A1, what is the highest quality audio standard it is capable of outputting in: (1) stereo analog; (2) multi-channel analog; (3) stereo digital; and (4) multi-channel digital. And the same for the HD-A2.
Multichannel digital over HDMI is always the highest possible quality output, regardless of player.
And which of these are available now, in practice, on HD-DVD media?
All are available now.
For instance, the spec sheet for the HD-A1 says it has 192 Khz/24 bit audio DACs. Does this mean that, if an HD-DVD comes with a DVD-Audio quality soundtrack (192 kHz/24 bit) that the Toshiba could decode it?
Dolby TrueHD is the same thing as DVD Audio, except for movie soundtracks. It's lossless and higher fidelity than you can get anywhere else.
It also says that the HD-A1 "supports" DD True HD in 2 channel only, but I don't know what the means.
All of the Toshiba HD-DVD players support Dolby TrueHD 5.1 at minimum. The web site indicates two channel support, because that was all the Toshiba players offered at initial release back in March/April. Toshiba added 5.1 support a few months ago via a firmware upgrade.
Finally, just how good is DD True HD? Is it CD-quality; or is it DVD-Audio quality? According to a 2004 press release from Dolby Labs, the highest quality audio format mandatory for HD-DVD is MLP Lossless, which allows for 24-bit/96 kHz multichannel or 24-bit/192 kHz stereo. So given this, why is everyone on this thread referring to DD True HD as the ultimate audio format for HD-DVD, rather than MLP Lossless? Are they the same?
Yes, Dolby TrueHD is the same as MLP lossess. "Dolby TrueHD" is the marketing name for MLP lossless in movie soundtracks, just like "DVD Audio" was the marketing name for MLP lossless audio on DVD music titles.
While TrueHD can support 5.1/7.1 24/96 as well as 24/196, you aren't going to see movies with such tracks, because 95% of movies are mastered with tracks of 24/48 or less. Eventually, we may see concerts and other music events on HD-DVD and Blu-ray with TrueHD 5.1/7.1 at lossless 24/96, though. Current Dolby TrueHD tracks are either 16/48 or 20/48 due to limitations of current workflow and muxing methods, but that is expected to change in the months ahead.