one more thing:
i think that "composer", "lyrics author", "engineer", etc. fields should be replaced with "music by", "lyrics by", "engineered by", etc. very often it happens that music or lyrics is written or engineered and produced not by a person, but by the whole band itself and it's pretty funny to see phrases like "Lyrics author - Cannibal Corpse".

besides, i think that there must be an additional field "written by" - sometimes you don't know who exactly wrote lyrics or music, like "Song Title - written by John/Paul/George/Ringo"