In pro audiobooks I've heard both approaches, and success depends a great deal on the individual actor/reader. On the whole, though, the very best of them rely a little bit on pitch and a LOT on intonation and vocal style to distinguish characters.
It helps a lot, of course, if the "voices" in the source material are clear, so that the words they use, and the rhythm of their speech is unique.
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It helps a lot, of course, if the "voices" in the source material are clear, so that the words they use, and the rhythm of their speech is unique.