These seem to have appeared with very little fanfare, but it's a huge improvement over any previous attempts at DVD Audio. Picked up the Bob Marley - Legend BD-A and it sounds amazing, and includes three different HD audio streams for those who enjoy picking out the difference between codecs.
Codecs make no difference, actually. Those HD tracks sound better coz they were remixed, encoded with higher bitrate and recorded using better hardware, that's all.
There's no remixing being done on any of these releases. They're sourced from the same 24-bit/96khz studio masters that the CD versions were sourced from. Regardless, the three different streams audibly have slightly different EQ, with the DTS-HD track having deeper bass than the PCM and True HD. Even though that was probably just done to justify including three streams in the first place