With the combination of the Node server and the Alexa skill you can ask Alexa to play your local mp3 files.
At the beginning, make sure to create a copy of .env.template and name it .env. The credentials you define in this file are used for a secure connection between Alexa and your Node server.
Run npm run build-alexa to prepare a package that can be uploaded as lambda function. You need to ZIP it manually due to a bug in gulp-zip.
Run npm run dev for development mode or npm run prod for production mode.