Zack said:No, you need to pack the rockwool between the studs and noggins before you fix the plasterboard.
However, if sound transmission is a real problem from bathroom to living room you always fix another sheet of plasterboard on top of the existing one then skim & redecorate etc. Gyproc and other manufacturers produce " Soundbloc" plasterboard (it is distinguished by a blue lining) which should drastically reduce sound transmission.
Z
From my experience gyproc sound block is not a satisfactory soundproofing system to be used. . The Irish Agrement board that certifies building products , have rescinded the certificates for these so called "sound resisitant plasterboard" because of recent problems in thousands of houses and apartments throughout the country.( see homebond circular and 5th edition of house building manual in bookshops) They have no sound resisitant properties.
Tip
The "composite fibrous wool backed plasterboards" are very good for applying to existing walls, as a means of improving soundproofing.
( brandname rockwall i think)
alternatively
a thoroughly packed layer of insulation between timber studs and boarded off with ordinary 12.5mm plasterboard usually does the job. However you must be thorough with the packing around perimeter of wall i.e along ceiling wall joints, along floor wall joints and wall - wall joints at the sides.
Warning
Putting more layers against a wall exacerbates a problem . Ive seen it in many houses. The problem stems from the air cavities gaps behind plasterboard where sound waves swirl around accumulate and ultimately amplify sound. so strip back the exisiting construction and get that packing in !