Q: How do I tell my iMac to use only the attached monitor, turning off the computer screen until needed? Having both on is distracting and a waste of energy.

There's not, just use the F1 key to reduce the brightness as much as possible.

I would have posted my reply here to esentially your same post in someone else's thread had I known you started your own thread.

The only addition you made here is asking about switching audio.  The basic answer is what was stated above, no.  However it can be done with the aid of SwitchResX and an applescript.

Here's the hint.

Change audio devices via shell script

When I enable my TV to watch videos using SwitchResX I have the applescript executed that also switches my sound output to the TV.  And when I disable the TV when I'm done the sound switches back to my computer. 

(And yes, my main monitor is on all that timem a 2nd monitor too -- sorry, I couldn't resist)