pat@ned you don't need any tools other than basic text editor like notepad or whatever.
Personally, I like working with examples, so here it is https://gist.github.com/rodydavis/d0cb7a53d8deb42e92ae803a9dd48dbc
You modify it to match your thing, make separate "item" section for every mp3 you're publishing and there you have it ;)
All GNU social JP content and data are available under the