Mary in Kentucky, In my opinion (but remember, this is strictly private opinion, nothing in this post is legal advice, nothing in this post establishes a lawyer-client relationship, etc., etc.) posting copyrighted music or lyrics on the web without permission is infringement, unless it happens to come under "fair use". (A simple example -- one of the few simple examples -- of fair use is a book reviewer quoting a passage from the book in his review.) I think posting MIDI files of copyrighted music would also be infringement. The question remains what specific right would be infringed by an unauthorized MIDI file. Is a MIDI file a "phonorecord" or a "public performance" -- or both ?
Your rule of thumb of avoiding music written in the last 50 years would have been almost right before 1978 when the term of copyright was 56 years. But beginning in 1978 the term of copyright was raised to 75 years, and in 1998 they raised it again to 95 years. What was published by the authority of its author in 1922 or earlier is now in the public domain, but nothing published in 1923 will enter the public domain until January 1, 2019.