I'd just keep it simple and just use one monitor's speakers as output and just leave the other one alone. No, it won't be center stage, but at this point I don't think we're all that worried about sound staging. If you turn up the speakers to about 80% and it's off to the left, it doesn't feel all that odd.
Doing that currently, but by mistake. Moved the main machine out in the livingroom to make room for a baby, fired up without the 5.1's, used HDMI, and sound came out...surprised me because I forgot! Anyway, since I barely get time to use it anymore, it's working just fine with the left monitor displaying sound. Like you said, just plop on headphones if you want to hear in a loud environment.
I also don't think the speakers are horrible. I can hear all I want to hear with the TV and stuff off, but good luck if any of it is. The volume ability is just not there. But they'll work just fine in the moments you have fairly good silence in the room.