HDMI sound is working using DP adapter to HDMI cable.
I had cable plugged in to DP2. As soon as I booted with it plugged into DP1, HDMI sound came right up.
I also have dual-monitors running using 2 DP adapters, but there are some glitches.
If the computer is booted with both monitors plugged in, it will reboot before it reaches the desktop.
If I plug 1 monitor into DP2, then boot and wait until startup is past the clover menu, then plug second monitor into DP1, dual-monitors work.
Maybe a glitch that can be smoothed out with a clover boot arg?
Dual-monitors are not important to me at this time, I just thought I would try them.