Midis

[Hoffenlarft]

  If you remade a song in a midi from zero, is it considered illegal?
  And if you put in the credits something like: "Song X remade a composed by Hoffenlarft"

[Anym]

Probably. Define what you mean by "remade".

In addition to the lyrics and the actual performance, the notes of a song are obviously copyrighted as well. So, if you create a MIDI from sheet music (starting from zero) and distribute it, that would be copyright infringement (if the sheet music isn't old enought to have entered the public domain already). If you remake a song by ear or from memory, it depends. If it ends up different enough to count as an artistic work in its own right or if it you use it in a way that could be considered fair use ("for purposes of criticism, comment, news reporting, teaching, scholarship, research,..."), it could be legal, otherwise it's copyright infringement just the same.

Crediting other people for their work is common courtesy, but just giving someone credit isn't enough to use copyrighted material.