Each environment has to be worked out for both inside noise level and surrounding outside noise level.
For most internal work the standard in offices for meeting rooms and interview rooms etc is the Blue/Purple plasterboard usually called Soundbloc MR.
You could put Thermaline boards onto the outer-facing walls this may also help.
As for technical or Regulations this may open your eyes a little...
This JUST deals with Ceiling Acoustics!!!
