Oubliettes

Yes, it's why i want use (old) classic music. Directly from partition (via .midi) and interpreted via a midi instrument (not a "direct" conversion of a CD or youtube rip).

No, it isn't. It depends on how the artist licensed the music etc. There are some websites where you can find royalty-free music, like http://freemusicarchive.org/genre/Electronic/?sort=track_date_published and https://incompetech.com/music/royalty-free/?

Be sure to follow the license, and if the creator requires to give credits, do so.

Just taking some random music and using that, even if in a non-commercial way, is generally not legal.

EDIT: especially with random CDs you have laying around, some record company holds the licenses and you are generally not allowed to use them, even in a non-commercial way (in a way without you earning money)

EDIT2: and with most classical pieces, some random record companies also hold the copyright for, if not the songs, the specific recordings. Be very careful with music. When in doubt, you are probably not allowed to use the track you are wanting to use

EDIT3: same goes for art.

This is why the Creative Commons license is so great for art&music, as it easily&quickly shows you what you can or can't do with that piece

EDIT4: maybe i'll add more edits, i don't know yet xD

Depend of the rights on the music. Someone give permission but by default, you can't.