This seems to suggest that while the Studio Display can be driven at 5K by a raw DisplayPort signal (which is great) the audio signal is routed over USB rather than the Displayport audio channel. That is unfortunate, I was hoping to at least be able to get video+audio.
Really I think the worst part of driving this monitor with plain displayport is that there's no way to control the brightness, just like the LG 5K.
I have a LED Cinema Display (DisplayPort model) on Windows that surprisingly responds to Monitorian brightness control when I use current AMD drivers. I used to run a specific brightness control app for Apple displays, but have since ditched it.