I understand that audio is voltage... I meant power, which we refer to as voltage in these situations. It would likely be +12V, or maybe +15V.
On any standard Whirlwind Q-Box that is used throughout the broadcast industry, there is an indicator light on top when it senses any voltage on the line, and no, it's not audio or digital voltage, but power. I'm sure there's a minimum amount of voltage needed to trigger the light to warn that there is something on the line that could damage some equipment. I just refer to it as voltage, since that's what most people are familiar with!