Or, if you've created a swift version anyway, use that codebase for a universal app?
Extract Audio Only. From there, you can also customize the quality of output files, convert to mp4, or send to Permute - a universal media converter. http://splendorsofnature.com/__media__/js/netsoltrademark.php?d=products-dd.com%2Fcategory%2F%25e0%25b9%2584%25e0%25b8%25a1%25e0%25b9%2582%25e0%25b8%2584%25e0%25b8%25a3%25e0%25b9%2580%25e0%25b8%25a7%25e0%25b8%259f%2F