Amazon Indian tribes, Andamanese, maybe remote communities of Bushmen, Aboriginals, etc.

I remember reading somewhere that Incas received commands from statues when the Spanish conquerred them. Not sure how reliable this information is, but I would count "hearing voices from statues" among the symptoms of schizophrenia, if that's true.

diagnosed with schizophrenia by DSM standards. And should we medicate them? The life of schizophrenics noticeably improves when they take their drugs.

The map (even a high-status one such as DSM) is not the territory. Asking "are they schizophrenic according to DSM" and "are they the kind of schizophrenic who is unable to function normally in their daily life" are two different questions. If someone hears voices which are completely benign, I'd say "live and let live". It's only the voices that make people cause harm to themselves and the others that should be treated by medication.

In the bicameral era, I can imagine that most people heard the relatively benign voices, and only a few ones heard the harmful voices. In other words, the actual problem of schizophrenia could be not hearing voices per se, but having those voices become dangerous. (Or hearing the voices so often that it makes normal functioning difficult; but how much that is would probably differ in the ancient times and now, especially when it's a social stigma now.)

The life of schizophrenics noticeably improves when they take their drugs.

Removing the dangerous voices improves life.

Removing rare and benign voices... I am not sure about that one. Actually, I could imagine this being the other way round, for example sometimes hearing the voices could manifest as increased "willpower" (e.g. it's easier to exercise every morning, if an irresistable voice of God keeps reminding you). Maybe akrasia correlates positively with atheism.

Meditation is not limited to the Hinduist or Buddhist religious context.

Then I'd cynically guess that people in those other contexts, if they meditate hard enough, usually receive hallucinations that confirm their contexts (e.g. instead of their previous reincarnation, they will see Jesus Christ or Holy Spirit or Allah coming and speaking to them).