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... ut non vox minax excideret, nihil per temeritatem proloqueretur?...

The "i" in the stem of the first verb is long, in the second short; the second fits the context with the meaning "fall out," that is. "let slip"; a threatening voice/utterance would slip out, so that the household should have been aware of the slave's threats. As a good lawyer, the speaker wants to show that the household is at least guilty of complicity in the master's murder.