Depending on your application there are two kinds of Flex duct used, the garden variety sold at the big boxes which have plastic inner sleeves. They are better than metal but not as good as true acoustical flex duct which has a fabric inner sleeve allowing the insulation to absorb some sound as it travels the length of the duct. The later is usually special order.
Make sure the duct is stretched out not bunched or crushed, wide turns, fasten well at both ends with collar ties and tape. Sometimes even a screw,