Changing the voice
Twenty-eight voices in Settings, four samples each to compare them by, and the one you pick speaks from the next read on.
Chippy speaks in one of twenty-eight voices, and the default is Carina. Open Settings from Chippy's menu in the menu bar, with Command and comma, or from the gear in the panel's header; the gear's tooltip names the voice in use.
Picking one
Under Voice, each row is a name and two words on how it sounds. Beside each name are four numbered buttons: every voice speaks the same four sentences, so number two in one voice is number two in every other, and you can compare them like for like. Press a number to hear it, press it again to stop. Playing a sample does not pick the voice.
Click the row to pick it. The pick is saved at once; there is no button to press, and closing Settings keeps it.
From the next read on
A voice is chosen once, at the moment a read begins, so the voice you pick speaks from your next read on. What is already recorded stays as it was: each recording keeps the voice it was made in, and History shows which one beside each row.
That has one consequence. Tap the right Option key on the same selection while that reading is still up and it replays as it was, in the old voice, because it is the same recording. Select something else, or the same text after a stop, and the new voice speaks it. The same text in a different voice is a new read, and both recordings stay in History.