One of my problems is now solved - the hitPlayer sound that used to cut out FOREVER was my own dumb fault - my player was DEAD after a while, and I have the sound set up to only play when he's alive, haha.
So that at least is not a Roku bug.
But now I've discovered something more troubling:
HDMI audio is less reliable than AV audio!
Roku 1 2710
AV audio - works reliably, all sounds play at correct volume, all sounds work, nothing broken.
HDMI audio - the first few (3 or 4) sounds don't play at all, every time. Then sounds start working, but volume adjusts sometimes for no reason, some play quiet, most normal, some silent (either 0 volume or not playing, can't tell), no pattern that I can tell.
This is all with my sound effect volumes all at 100. I tried setting all volumes to 50, same thing happens, first few sounds don't play, then they play but vary in volume randomly, most are normal, but every so often one is quiet or silent.