The audio room is in the basement?
Garage floor and audio room floor are approximately on the same elevation?
Common garage wall is unfinished concrete?
All walls and ceiling of the audio room are finished drywall?
Three walls of the audio room are concrete furred out with studs? Are you sure they are studs and not 3/4" furring strips? If studs are they 2" x 4 1/2" nominal studs or possibly 2"x 2" nominal? (1 1/2" x 1 1/2")
If they are indeed 2x4s do you know if possibly they were spaced out from the concrete walls by at least 1" or 2"? Hopefully so. You can check this by removing a wall plate from a communication wall outlet, if there is one in the room. Otherwise remove the cover plate from a duplex receptacle. Take a metal close hanger and straighten it out or cut out/off the long part of the hanger to use as a depth probe. If you are lucky the communication box may be an open back wall communication bracket. Then you can physically look inside. If not make a hole against the side of wall rough in box, side opposite the stud side the box is supported to. Push the wire through the hole made in the drywall to the concrete wall behind. Mark the front of the wire probe. Measure the length.