we converted our double garage 18 months ago into a playroom. We removed the garage doors and put 2 windows in and I had them made to the same size as the dining room window which is the other room on the front of the house. That way it looked less like a garage conversion.
We retained the back 1.5m of both garages as a store. We had to apply for planning and building regs and it took 3 weeks. We had 2 radiators put in, a block wall inside to create the store area, knocked through from the hall, had flooring laid, loft hatch closed up and relocated (double garage where only one of the garages has a room above it so we have storage above) strip lighting moved into store area and downlighters put into playroom.
The final room is about 5.5m long and 3.6m wide. We have put 4m of wardrobes in across the back wall, there is an internal door into the store. We also had new flooring laid in the hall and dining room. Cost IIRC was £12k
So, if you are converting just one garage and having it as a cinema are you intending to retain the garage door at the front? You can use this as a little storage area and put a wall up behind with enough room to allow the garage door to open. That way you can keep your room very dark.
There are lots of AV forums where people create home cinemas and the window coverings are always an issue when trying to cut out all the light.
You can get trunking put inside the wall to enable you to run cables easily, plus there are either plates with all the connections on so the wires are attached to that and you just plug stuff in or you can get a brush plate (like a letterbox) and run the cable out of that.
The most important thing is insulation, go overboard on it so that it isn't freezing.
Wow, that was long 