So this setup could be built actually fairly simply - no need to have phantom power or anything like that. Too complicated.
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Going back to the kind of electret capsules that kick-started the mic DIY boom, you should just be able to feed them all from a 9v battery through something like a 10k resistor to each.? Then collect your audio output through a nice big capacitor (to strip off the DC), and you've got something easily compatible with your recorder's inputs.? Unbalanced, but over a few inches or feet it won't matter.? You can do your custom summing with resistors and/or potentiometers.? There will be a little interaction, but something you will be able to cope with.
