Which of these are correct?
In a frail voice, he said goodbye.
With a frail voice, he said goodbye.
This is more usage than grammar; grammatically, both are correct. I'd say "in a frail voice" would be more common usage.
They are both correct. The first means that his voice was frail when he said goodbye, while the second means that he used a frail voice to say goodbye, so essentially they mean the same.