We have Wikisource for the public domain sheet music (example La Marseillaise)
Basic Idea 
To gather sheet music and lyrics in different languages much as Wikibooks collects books.
Hopefully people will add sheet music that resides in the public domain, such as classical pieces, or they could score popular pieces themselves which may remove any copyrighting concerns.
The Biggest Problem 
We would need to develop a scoring interface similar to the mathematical formula formatting (LaTeX), or maybe a new layer on the normal Wiki interface.
Discussion over this, and of the possibilities for music markup in all media wiki projects (wikipedia, wikisource and wikibooks, specially) is going on at Music markup.
Music is a vast body of knowledge which should be freely available so that people can study it. Sheet music is a low tech and data efficient way to store musical knowledge. Currently, sheet music is inordinately expensive due to distribution costs. The Internet would remove this cost. The scoring interface could also be invaluable in the future for other open source and wiki projects.
- Music markup
- Wikimusic II
- Music notation on Wikisophia
- Musical notation codes and programs supporting them
- Music Markup Language is an XML format that needs much microparsing. One supporting Java Applet is known.
- Music notation markup languages a comprehensive breakdown of the competing languages on coverpages.org
- Lilypond an open-source program "for making beautiful music notation." and a music notation format
- Project Gutenberg - digitising public domain sheet music (inactive)
- IMSLP International Music Score Library Project (Wiki)
- Wiki-score Platform - Cooperative work on large score editing
- Mutopia Project - Free public domain sheet music
- CPDL Choral Public Domain Library, free public domain choral works (now a wiki)
- Musipedia : The Open Music Encyclopedia , a searchable, editable, and expandable collection of tunes, melodies, and musical themes.
- brainstorm about a Wikimusic