Talk to Steve Mattie at Transport Designs Inc. http://www.transportdesigns.com/
They are located in Montoursville, Pennsylvania and only do custom work; all to your requirements.
I'm not sure if they have built a recording studio, but they have just about built everything else!
I will say that Transport Designs built my "custom" truck-conversion motorhome in 2001 and after 115,000 miles it is still going strong.
I have no financial interest in Transport Designs, just a very satisfied customer!