Thanks everyone for the replies. The issue doesn't appear to be related directly to sleep/power saving mode.
There seems to be a small number of variables that I can toggle: Mac power status (on/sleep/power-save/awake), Helix power status (on/off), Helix USB plug status (already in / subsequently plugged in) and Sound Output selection status.
Since reading the suggestions regarding the Mac power status, I've tried a small number of different variations. In each case, the Helix USB was available in the sound output preferences window.
So, I'm still unsure of the conditions that make the Helix USB not show up as a sound preference. As soon as I figure it out, I'll add a comment. I'll also (casually) continue to look into what can be done to force the Helix USB to be recognized, through Terminal or other means.
Again, thanks for the replies and suggestions.