He actually didn't speak through his snakes, he was inside of his snakes before he slipped out of them:
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Here he speaks "through" the snakes while slipping out of them.
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He spoke out of the snakes while being inside until he slipped out.
You can't really compare Kabutos snakes to Nagatos summons because the snakes are living things while Nagatos summons are not. Nagato share his vision by giving his summons the Rinnegan (his own eye).