Just a note for anyone else starting with the rock64 DAC. As shipped, it came with two jumpers on it. J3 was set to link the middle pin to MCLK (look at the silkscreen on the reverse side) and J4 was connecting the middle pin to L (any ideas what that one is for?). I wasn't getting any sound from volumio with a clean install of the latest release to eMMC and the ouput set to I2C. I saw a reference on the forum elsewhere to moving the J3 jumper to link the middle pin to 50M (use the onboard 50Mhz oscillator rather than an external one?) and voilà, there was sound! I did it whilst the rock64 was powered off... I wouldn't recommend doing it whilst it is running
That minor hiccup aside, it is both volumio 2.390 and the DAC are working great and sound good.
I'm guessing the SPDIF is working fine also as the port does light up if I change the audio output to SPDIF, but I don't have a SPDIF amp handy. I remembered the MiniDisc units I have have optical in via 3.5mm jack, and let you monitor the output. I switched volumio back to SPDIF, connected the MiniDisc up, and clicked on the next track to play and it also works perfectly! Great work guys!
Now to work out how to get access to the rPi GPIOs as unfortunately the pine64 guys didn't think of laying out the board in a way that they could be extended through (but just not populated) and whilst the board would fit perfectly with standoffs the link to DAC board to the rock64 board, it does block off the rPi header... might have to make a board that stacks in between and allows access to the rpi gpio via 90 degree headers...