HVAC Recommendations

Hello! I'm just in the HVAC planning phase for a new control room build and was hoping to get some advice on my options - and thanks in advance! The build is in an old (1880's) brick and beam warehouse space on the ground floor. My control room will essentially divide a 1000 sqf room in two, with the existing HVAC supply (all exposed 14" round duct work) running parallel to the exterior of my wall. The supply is the end of the line for the building, the duct work runs through several other suites before mine. There is no return ductwork, all the other offices simply have a hole in the wall with a grill. The contractor has suggested drawing a supply line off of the main duct and the "hole in the wall to nowhere" type return, which is obviously not the ideal solution. I've considered a few ideas, such as building a baffled plenum for the supply but haven't really come up with anything concrete. My main concern is how to adequately provide cold air return for the room without compromising sound isolation, as much as possible. Is simply cutting a 12" hole (sound issues aside) even an adequate air return method? If so I could try cutting a hole at the top of the interior drywall layer between two studs, leaving the cavity open, and cutting a hole at the bottom of the exterior drywall. Would this even work without a fan to suck air out of the room? Anyway my appologies for my lack of general HVAC knowledge, I'm just trying to prepare my options to a contractor who sees no problem in cutting a 12" hole through a sound studio wall haha... Here's a pic of the layout - and once again, thanks in advance!

My main concern is how to adequately provide cold air return for the room without compromising sound isolation, as much as possible.

Silencer boxes. That's what you need. One at each point where the duct penetrates a wall. It's just a large box lined with duct liner, and with several baffles partially across the interior, forcing the air to make many turns as it flows through. Search the forum and you'll see many designs for these silencers.

Is simply cutting a 12" hole (sound issues aside) even an adequate air return method?

Nope! Not unless you put silencers on it, and a fan, and confirm that there really is a good return path to the HVAC system, and also that 12" will give you the correct flow volume and speed for your room. Speed must be low and volume must be high. - Stuart -