or 
I saw this electric guitar (Age: 3-8 years) at KbToys that would make a nice decor and/or functional item. With the five pieces included in this Electronic Guitar Combo Set, you're ready to begin playing and singing right away. Put the microphone on the stand or use the headset microphone for greater mobility.
They both plug in to the base of the microphone stand that doubles as the amplifier. Adjust the stand so it's just right for you. Find the right volume, strap on the guitar and launch your brilliant career. Colors may vary.
You could even pick up a cheap used one. Paint it a color or colors that will accent your room, decoupage on it, or stencil it. Anything goes! Almost cheaper than wall hangings.
Of course there are all the old records and other music collectibles that you could hang on the wall. Save all those free internet access trial cd's you get in the mail. Put them up on the wall. Drill Holes in them and string them together with fishing line and make mobiles or large curtains. They reflect the light really well.
You could even glue them onto ugly pieces of furniture that you base coated with your favorite color. String them through their center holes with a large bead in between each one, having sections with several beads or a piece of bamboo or something for a spacer.
Is this cool or what? Just came across this Piano Keys Rug
. And it's a synthetic, which means easy care.
Search Google for more Music Theme Ideas
Strobe lights, bubble machines and disco balls would add to the ambiance of a music room, especially if you're going for the nightclub or rock and roll twist. Here are Discount Prices on Fog & Bubble Machines lava lamps, black lights and more. There's even a 5' x 11' sheet music mural of Lionel Ritchies Endless Love! You could put the 5' or 6' Sax or Trumpet player silhouettes on your wall, or stand them up with an up light casting their shadows on the wall.
And for your little Diva, this Stage and Microphone Stand from shindigz.com that will add to that music theme room. You can get the stage, the feather boas, costumes and inflatable instruments. Hang the musical note door curtain over a window or against the wall for accent.The inflatable instruments could be left flat and hung on the wall, or inflated and suspended from the ceiling out of reach.
Keep in mind that a lot of this stuff is meant for music theme party decor, and not for long term survival. But if you're careful about how you use it, it should last until the next "mood" strikes!