Ok then gents. Headroom and supply voltage 101 coming up!
Take a very familar device, the humble guitar pedal. These normally use a 9volt PP3 battery. I shall take that as 10volts cos' it nearly is with a fresh batt and it makes my head hurt less.
The absolute maximum voltage that pedal could put out is 10volts Peak to Peak. Think of a sine wave with the positive peak just at 10V and the negative at 0volts, of course it could, probably would clip, but the P-P voltage still cannot exceed 10V.
But, all the signal voltages you ever read about are "rms" values. That is 1/2 the P-P voltage times 0.707, thus the rms maximum would be 3.535volts...BUT! The electronics can never hit the internal rails and so we would be lucky to get 8V P-P giving us a resultant rms output of 2.828 volts. If we double the supply to 18volts we would double our max rms output to 5.656V (actually we might do a bit better than that, depends on the circuit devices)
But we must be careful in the case of the electret mic. It might well be that they all run on the lower voltage and a dropper resistor, possibly also a Zener is used to provided higher ratings. The signal headroom would then be just the same.
On a related subject the poor results some folks report from cheaper gear is often a function of lower internal supplies. "Pro" kit generally runs op amps at +18-0-18 volts but as you drop volts you drop costs, (caps, transformers) so many cheaper mixer e.g. have +&-12volt rails. This does no harm so long as you don't drive them too hard!
And you don't HAVE to use XLR for phantom power, any 3pole circuit will do. NOT advised! But it will work. You don't of course need spook juice for an "electret" mic!
Dave.