Final answer:
To avoid repeating an idea, the sentence can be revised to 'Their voices swell as they begin singing a folk song,' maintaining clarity without unnecessary repetition.
Step-by-step explanation:
To revise sentence 2 and avoid repeating the idea, the sentence can be made more concise and direct while maintaining its original meaning. The best way to revise the sentence would be to remove redundant elements such as 'begin to start' and simplify the structure to convey the intended action clearly. Therefore, the revised sentence could read: 'Their voices swell as they begin singing a folk song.' This revision maintains the core idea of the original sentence, which is the swelling of voices as singing commences, without unnecessary repetition of the concept of starting to sing.