I have the same box (only difference is 16 vs 8 GB storage), sound over HDMI works fine for me. What device tree did you choose?
gxl_p212_2g.dtb is the right one for you!
Thanks for that info, I will try that tree ...
I used the default tree from the image, as it was a fresh install and the install instructions noted:
" S905X: Device trees for these boxes start with gxl. You can leave a tree that is already present on SD card (fresh install only) or choose one according to your RAM amount. "