I know midi's are composed by individuals but when they make a midi which is just a song they found and they make it sound different, isn't that copyright infringement? If it is copyright infringement, then how are ther so many sites that offer midi's?

not all the sites r legal

if they make it different then how is it infringement.

i seen a post like this some were and i think midis are open to everyone

yeh, i think theyre open source no? just kidding, thy can be copyrighted and even if you modify ms office its still illegal. unless its linux