Although the link below will take you to many great ideas for bulletin boards in the *music* classroom, it might also offer some general inspiration for a music-themed board in a regular classroom. Without knowing what level you teach and the background of your students, it's hard to offer specific ideas.
One general idea that does come to mind is "Music Makes the World go 'Round!" It could include a big graphic of our world with strings going out from different locations to graphics of instruments and performers from these areas.....eg. string from Andes Mt. region of South America to a picture of pan pipes. (real would be better!).....from Japan to a koto..... If there are specific regions/countries you include in your social studies curriculum, I'd certainly include those.
I love making music-theme BBs! Here are a couple of ideas - hope I can describe them well enough:
1. I like to do a bright background (yellow or blue maybe). Make scale with black paper. The notes are actually flowers kind of "upside down". The bloom is on a line or space, the stem goes up, add a leaf or 2 at the top. You can have quarter notes, eighth notes, etc., depending on the # of leaves on each flower. Scatter more flowers here and there on the BB. Title I used was "March into Spring ... and SING!"
(exclamation mark was also a flower :-)
2. The entire BB is a grand piano: start with a white background. Lay out keys along the bottom with black paper for the keys and the tiny spaces between keys. Basically the entire right side is a sweeping curve of black paper in the shape of a grand piano. I put the title in black letters in a curve along the piano: Music is a GRAND adventure!
3. I like the "Music in our Schools" theme of "Connect with Music" - I did a multi-disciplinary BB once with "Connect with Music" sign in the center. Music-themed fabric as background. Then all around the BB, I put headings of different subjects: math, science, art, history, etc. Under each heading I had pictures, photos, charts, CD covers, etc. that showed how music could be found in all these areas. Then I "connected" them all back to the center sign with a length of yarn. It was very effective at showing how all these areas are inter-related.
I hope these ideas spark something for you!