Free will.
It isn't simple introspection. It is a rigorous scientific experiment, Descartes' evil demon thought experiment.
Additionally, you can only prove your own consciousness exists, not consciousness in general. But that's OK, classic materialism will suffice to determine that the world probably isn't an illusion and other beings like you probably experience similar phenomena.
Meditation is not a rigorous experiment, if someone can formulate a reproducible experiment like Descartes' thought experiments that produces this phenomenon then there's something. Further, there's no requirement that a voice be a singular one for the self or consciousness to exist. We can deduce that they exist, how they work or what exactly they are is not known.