Cheap visual indicator for a PA loudspeaker line
Many possibilities.
 - How much do you want to spend per location and  


Have you got AC or DC power at each point? 


IF this was line powered,what maximum power can you afford or want maximum to be taken per transformer from the audio signal for driving the inicator (eh 100 mW, 10 mW, 1 mW ...?)



Neon:             The classic easy and cheap approach would be to use a Neon bulb and  a resistor  Would take some fine tuning in the design but not hard, line powered and there will be many existing circuits available. Note that, as Olin says, and as per graph at ane, for Voltages below 100v it will not always "strike".

LED     Something using an LED will also work and be easy to do.
You can easily produce a low voltage signal that varied with speech etc and which can drive an LED 'in time' with the sound, Or you modify the same circuit to light an LED steadily as long as there more than a certain voltage present.   


Component count for a steady or flashing indicator can be
Line1 - Diode-|>|- Resistor - LED -|>|- Line 2     

LED flashes with AC
As above but add capacitor across LED
LED steady
Tap resistor add zener from centrepoint of resistors to line 2
Adds protection against overvoltage.


A 100 VAC RMS line will lose 100 mW for every mA drawn. A 1 mA driven LED indicator can be easily "good enough" for the purpose. 


eg as an extreme example a Nichia "Raijin" white LED (about as efficient as any on sale on earth) operated at 1 mA will produce about 0.5 lumen of light - which will illuminate a one inch square to about 600 candella or about twice the brightness of a typical modern  LCD  screen at full brightness. 


A VU/bar / dot meter type display also easy subject to powering answers. VU meter using analog meter is passive. 



Minimum "Neon" strike voltage

Paschen curves.
 Discussion here