Hi Jim,
We've done some testing here and are not able to replicate the problem. We are able to hear the AOA output, even when the EFIS is powered only on the internal battery.
For the AOA alarm to sound there are a couple of requirements. First, the AOA needs to have been calibrated. Next, airspeed needs to be present as well. Also, if the audio input that your EFIS's output is connected to requires power and your aircraft's power is off, the AOA alarm may not be heard simply because that audio device is not on. How are you testing the AOA output on battery?
Our EFIS-D10A installation guide does not reflect this yet, but for volume control you can hook up a variable resistor as described in our EMS-D10 installation guide. Their audio outputs are effectively the same (except for the pin-out):
"The intercom audio alert connection, utilized during
alarm conditions, should be wired to a 10k-Ohm
variable resistor (potentiometer). Ensure that it is
connected similar to the following diagram. You may obtain the variable resistor from Radio
Shack (part # 271-1715) or other electronics supplier."