I don't remember the exact details, but "massive stability issues and unfinished content" does sound like a pretty compelling reason to not go with that version. Presumably version 16 was what added the new stuff and 17, 18 and 19 were bugfixes (I have no surviving documentation from those days, it was before I even started using source control).
sound<NUMBER> is the default filename for a new sound asset before it gets properly named, so it was something I added and immediately forgot about. Back in those days I usually had songs on repeat in the GM editor instead of having a separate media player / youtube playlist for music so it might be an intentionally unnamed song I just replaced whenever I wanted to swap it out.