I have an idea i am sure someone has thought of – if you know of the progaram let me know
A program that lets me input a music sheet or guitar tabs or some way of identifying music (e.g. somethin like 2C 4D 1R for a half note at the C tone, quarter note at the D tone, whole rest, etc)
and it will play the music on piano or flute or whatever?
Ever heard of such a thing?
I really only understand about half of what you want, however, MuseScore is a fairly powerful free program, that can do some of the things that you would like.
However, I don’t think it can input a music sheet without you entering each note. Or giving it the time signature, etc and playing it into a midi input with a keyboard… I don’t think it has playback in anything besides piano either.
Take a look at the website, and it will explain what it can do better than I can… 
http://www.musescore.org
Almost Any DAW does that. I use GarageBand and I can input notes.
Some of the others that program (MuseScore) mentioned were exactly it (Finale and Sonique) so Thanks! N00b! (and you too TWS)
You are welcome
Glad to be of service… 
MuseScore is a schweeeeet program. Hard to believe it’s free.
If you want to build really good-looking sheet music, and/or to prepare an input file for a professional-grade music engraving program, “you just found it.”