(windows) command line m4a tagger?

[simoncn]

Thanks for the test file. The new version of MinimServer (not yet released) is reading the multivalued SINGER tag correctly:

SINGER=Christopher Purves
SINGER=Roberta Invernizzi
SINGER=Sonia Prina

I'm not sure why you think mp4 tags are in such a mess. The spec supports multiple values for all tag names and there are taggers and servers that correctly implement the spec.