When you upload MP3 files into AWS S3, they will default to being a content-type that browsers will treat as a download (I believe “octet-stream”). If you want to change this so the delivery will be streaming, you need to change the content-type to “audio/mpeg” via the AWS console by editing “Properties” -> “Metadata”:
And if you want to revert back to forcing download, just remove the content-type metadata — note that there is no pulldown value for octet-stream!