Open-source is irrelevant here. It's (1) "I own this copy of music/software and can do anything I wish with it" vs (2) "I licensed this copy of music/software from you on the terms we have agreed on, and I will adhere to these terms".
Both music and software are usually licensed (2), but people often disrespect music license thinking they are entitled to more rights for some reason.
As far as I know, sheet music is usually licensed under traditional copyright and remixing/sampling is usually licensed under fairly complicated closed licensing terms, for the cases where samples & remixes are cleared legitimately.
Creative Commons licensing of sheet music & samples would count, but I don't think that's what the parent comment was alluding to - it was a misunderstanding between the concept of "open sourcing" something and the concept of licensing it from the author under more restrictive traditional copyright terms.
Wow, this is a lot less snappy when you have to spell it out.
