What are the differences between the various mp3 test cases?

This should probably go into a separate file, togheter with the inverse transform.

This can then be tested independently, i.e.

for rating in {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10} {
  r = wmpRating(rating);
  Q_ASSERT(rating = balooRating(r)); }

It tests all possible ratings, since the commonly used numbers are somewhat arbitrary, and I wanted to make sure writing and extracting works

We have to test the rest anyway, so I don't see much benefit in this honestly.

Is this the same as

auto id3Tags = dynamic_cast<TagLib::ID3v2::Tag*>(file.tag());
if (id3Tags) { ... }

?

leftover ...

No. Mp3 supports ID3v1, ID3v2 or Ape tags, so simply casting to ID3v2 is not possible.

but in case file.tag() returns an ApeTag, the dynamic_cast will return a nullptr. This is a dynamic_cast, not a static_cast.

It always returns a nullptr

This requires a range check here - and then you can use a plain array for lookup.

I think this should be split in
writeApeTags(Taglib::APE::Tag *tags, PropertyMap) { ...; tags->setItem(...) }
and
writeXiphTags(Taglib::Ogg::XiphComment *tags, PropertyMap) {...; tags->addField(..., replace) }

Thats much more in line with the other calls.

Remove ...

missing spaces

please add an end of anonymous namespace comment here, it is hard to spot otherwise