Do NOT use green wallboard in a tub/shower area. Most building codes no longer allow it to be used in areas that are constantly wet. It does not stand up well in the event that it becomes saturated, and it will support the growth of mold. You're on the right track by using Durock (or similar cementitous backer boards.) Greenboard can be used in other non-wet areas that are subject to high humidity conditions.
There's no requirement (USA) to put anything behind the durock located over your tub. If you're planning on using it (not already installed) I would consider switching to green board. It is much cheaper and most of all, lighter. Green board is only sheetrock that has been waterproofed - and it works well. If you are installing durock, ensure you securely attach it to the joists above the tub. Also, if installing any lights, fans etc over the tub, the sheetrock is much easier to cut and affix fixtures. Good luck
You need to attach the durock to the studs behind it. I am assuming you will then cover the durock with tile and grout and caulk it, that will keep the moisture out of the walls. Nothing has to cover the stud walls behind unless this is to be a steam shower (much more prep will be needed)
If the tub wall is on an outside wall it should be insulated & then sealed with plastic, then durock, then the tile. For sound purposes you can insulate the other walls also.
There has to be done more then that. 1st I would check your local codes 1st some say yes some say no. But when you have the wall apart you should insulate and put plastic up. This will do a few things It will deadin the sound and if you ever put in a steamer or anything else the walls will be preped. Plus doing this will not cost you much at all. Maybe 100bucks if that. Its much better to do it now then later Plus if you sell your house Its a good thig to say. Hey if you want to do this or that The walls are ready for whatever you want to do. Im a Plumber for 20yrs and everytime I run into this and I tell them sorry I cant put in a steamer or other things cuz it aint up tp code and so on. So take the extra time and money and do it correct. If you just want to doit quick and not care then use d-rock make sure you do the seams and the CORRECT SCREWS!!! Goodluck
