Final answer:
The literary device present in the sentences is an oxymoron.
Step-by-step explanation:
The literary device present in the sentences is a literary device called oxymoron. An oxymoron is a figure of speech that combines contradictory terms. In the first sentence, 'They spoke without a voice,' the contradictory terms are 'spoke' and 'without a voice.' In the second sentence, 'they leaped without a sound,' the contradictory terms are 'leaped' and 'without a sound.'