FWIW, it seems that this is an OSX or perhaps app limitation and not a
Mini limitation. My 2010 Mini is a dual boot system with Win7, 64bit
in a Bootcamp partition. I boot into Win7 if I want to watch BluRays
with TrueHD or DTS-MA and use PowerDVD. It has the codecs
necessary to decode these formats. The result is, in a Windows
environment, the resulting audio is 24bit, 192 kHz. It also will
upsample other formats to this bitrate. The choice is also available
in Device Manager.
So, the Mini hardware does support 24/192, it seems OSX or perhaps
the API for apps doesn't. iTunes itself definately only supports up to
Apple take the point of view that with digital audio connections which includes HDMI, the final device in the chain i.e. the actual device that outputs the audio sound should be responsible for controlling audio settings including volume. This is because if you alter the signal at the Mac end you are actually changing the digital values before they get sent out the HDMI connection and Apple's (presumed) view is they don't want to be guilty of degraded the signal by altering it.
Saying this, it is in theory possible to have the first device in the chain i.e. the Mac mini still send the digital audio signal unaltered but also to send separate commands down the HDMI link to tell the final device in the chain to adjust the volume on behalf of the Mac mini. This is via a standard called CEC - Consumer Electronics Control, unfortunately Apple do not support this standard.
There is a device which can 'insert' CEC signals in to a HDMI connection, see http://www.pulse-eight.com/store/products/104-usb-hdmi-cec-adapter.aspx
I am puzzled too. My friend's 2010 Mac Mini does 192/24 over HDMI on Maverics like it means it. My acquired last year late 2012 Mac Mini does only 96/24. OK, I downgraded from Mac Pro, which had a 5440 card and used to output 192/24, and I CAN HEAR THE DIFFERENCE!!!
It's common knowledge that Apple people officially do not read this board (at least they do not respond to posts). So, this is a cry for help to whoever has enough knowledge to tell whether this is a newly introduced hardware limitation, or something Maverics needs to be hacked into. Please?