If I where going to build a room from the ground up for tracking and mixing hiphop & RnB what would be a good starting point?
Your local Building Inspection Department. Thats why every studio is different. They are in different locations, with their own set of conditions, restrictions, budgets, priorities, goals. Without knowing what your restrictions are, specific to your location and property, you can design anything you want, but that doesn't mean you can build it. You see, law is the primary designer. By virtue of CODES and APPROVAL of your design by issuance of permits to build. If you don't know what the restrictions are, how can you design the building. In other words, how much time do you have to RESUBMIT your plans untill they are approved.
IF you are talking about a personal studio on your own property, the process can be easy or it can be difficult, depending on many factors. Mainly from the gitgo, how familiar you are with the permiting process and construction. If you arn't, I would suggest enlisting the services of a professional in this field. It could be an architect, building designer or contractor. However, being as this is a studio, you need to give them your criteria for your goals. That the next piece of the puzzle. But you can't solve all of them untill you know WHAT the restrictions are. There are all kinds of them. Height, easements, setbacks, soils, parking, square footage, egress, electrical, ventilation, and many more.
What most people don't understand is, STUDIO design from the ground up is NOT normal construction, and because of safety codes, not to mention construcion, you are subject to inspection of every part of the construction, from foundation to roof, as it is completed. This means, there is a CONFLICT between SAFETY and TRANSMISSION LOSS. Let me give you an example.
Depending on isolation goals, current partitiion and exterior wall STUDIO construction usually mandates the use of DOUBLE Wall, two leaf, MASS AIR MASS construction technique. A typical home exterior wall or partition is a SINGLE WALL, two leaf, mass air mass assembly. Typical UBC(Unified Building Code) calls for a frame whereby BOTH faces of the frame are shiethed in a gysumboard(leaf) that meets local code regulated fireratings. Within the airgap between these leafs, and between each pair of studs, usually, a firestop is required, which is simply a 2x4 blocking nailed to the studs, midway up the airgap, usually 48" from the floor(for edgnailing the drywall). This keeps fire confined in a space with litte or no air leakage. These two code requirements guarantee the wall meets a minimum fire rating.
However, how would one "firestop" an interior DOUBLE wall, two leaf assembly? Since a Double walls purpose is to DECOUPLE one leaf from the other, If you connect the two walls with ANYTHING that acts as a firestop, you are COUPLING the two leafs, thereby negating the whole purpose of the design. See what I mean? And there are literally tons of these type of conundrum when it comes to studio design. Like double doors.
You can draw all kind of ideas. I mean the net is SATURATED on studio design forums with every kind of design imaginable. The probability of most of them recieving a permit on the first submittion to actually build them is probably close to NIL.

At least in the counties I have lived in. If you don't believe me, give this a read to see why.
http://www.johnlsayers.com/phpBB2/viewtopic.php?t=2921&start=0&sid=15cd0df6b8c862c4098369cacd67c8fd
Needless to say, acoustical and isolation concern solutions are all over the net. What ISN"T, is YOUR set of conditions, location, zone, etc, relative to these solutions. Thats why, before you even set a pencil to paper, you might want to visit your local BID and get a copy of a typical residential restriction sheet. Most BID offices have them on the counter or ask the clerk. Once you know the basic restricitions, then you can see your physical limitations. Then you can address specific issues.
Last bit of advice. Learn patience. And read read read.
fitZ