Works of authorship(such as music and software) should be free, and by freedom we mean:
the freedom to use the work and enjoy the benefits of using it
the freedom to study the work and to apply knowledge acquired from it
the freedom to make and redistribute copies, in whole or in part, of the information or expression
the freedom to make changes and improvements, and to distribute derivative works
Hatsune Miku is a proprietery soundfont for the proprietery VOCALOID software.
VOCALOID is a singing synthesizer based on sample concatenation.
Users of VOCALOID often share there their works with others, sometimes under Creative Commons licenses.
Because VOCALOID is so popular, Ameya/Ayame created UTAU that allows users to record their own samples.
Unlike VOCALOID, UTAU can be used with freely licenced samples and a free synthesizer backend.
But the frontend written in Microsoft Visual Basic is propriertery.
Richard Stallman, founder of the GNU Project and the Free Software Foundation believes that software should be free.
When he speaks of “free software”, he’s talking about freedom, not price.
He co-authored the GNU General Public Licenses that are intended to guarantee your freedom to share.
If one of the four freedoms is missing the program is nonfree. Copyleft guarantees that a free program remains free.
To understand how a program works you need more than just the compiled computer code; you also need access to the human-readable source code.
Some people call this “Open Source” but Open Source misses the point of Free Software.
Matt Lee, one of the authors of GNU FM, is the campaigns manager at the Free Software Foundation.
He also thinks that music should be free. The Creative Commons Attribution ShareAlike license is a copyleft license that can be used for music.
A recording of music made with physical instruments such as a human voice or an acoustic guitar can distributed music under a free license under a free format such as Ogg Vorbis. In this case there is no source code for the music. When using software for making music there is a source code that is part of that work. So it might be better to share your work under the GNU GPL. There should be no restrictions that prevent you from sharing soundfonts and using them with free software.