Mapping function keys to control volume

I've got FunctionFlip.prefPane swapping my F7,F8,F9 keys. I can see the proper escape sequences being typed in a terminal window when I press them.

I would like to remap F8,F9 to be Volume Down, Volume Up.

In the Keyboard Maestro editor I have:

Triggered by any of the following:
This device key:
Apple Internal Keyboard/Trackpad F8
ignoring modifiers
Will execute the following actions:
Decrease system sound volume

likewise for F9/Increase. Both enabled.
I can trigger them in the Trigger Macro by Name.
I can see them both execute in the debugger.

But I don't hear the volume change!
Any clues? It sure seems like a n00b error!

If you simulate the hardware keys to increase/decrease the volume, the on screen display should show the volume change, but there is no sound associated with that change (I just checked on a laptop and the result is the same there with actual keys - no sound when you change the volume).