https://www.google.co.uk/search?q=site%3Ahe...ag.de+MP3TAGAPP shows
https://docs.mp3tag.de/scripting
$getenv(x) returns the value of the environment variable x.
is missing
http://www.mp3tag.de/en/changelog.html
[2011-04-18] NEW: support for inbuilt parameters MP3TAGAPP and MP3TAGAPPDATA for $getEnv.