While I thoroughly enjoyed the ideas already presented thus far, I guess there are some parameters that need to be noted.
What are you looking to do with this music room?
Have fabulous stereo for listening enjoyment?
A room for full band jamming with drum kits and all?
A place for relaxing acoustic jams and practice?
And the other use of the room?
How often are guests expected?
Do you need a full or queen bed in the room?
Is it to be set up all the time (Futon, Murphy bed or regular)?
Do you want to move things around when someone does crash there?
The topic is of interest to me as my girls are grown and gone and we are going to retro fit the two bedrooms (similar size to yours), one for a visitors bedroom and likely a music room for practicing, no jamming but with a Murphy bed or a futon for overflow (both kids and significant others for holiday visits and such).
And I love egg cartons and their purported great sound abatement properties but I have yet to come across an actual implementation. Seems most folks don't want that particular decor.