I can definitely understand you cringing about the re-encoding, but I just wanted to point out that the reason that we took this approach was simply to give a semi-plausible real-world example of how tools like aws-serverless-express [1] and Exodus [2] can be used to build useful APIs with AWS Lambda [3] and AWS Gateway [4]. These articles are primarily meant to be educational tutorials that people can use as a reference when writing and deploying their own APIs. The whole "converting to MP3" thing is just an easily understandable premise which lays out a clear goal for the tutorials to build upon.
[1] - https://github.com/awslabs/aws-serverless-express
[2] - https://github.com/intoli/exodus
[3] - https://aws.amazon.com/lambda/
[4] - https://aws.amazon.com/api-gateway/