As already said, to a large extent sound proof also means airtight. This means sealling all the joints! Its, worth noting that if you build a 6 x 2m stud wall but leave a 1mm gap all round its like having a 144 (5.5") diameter hole in the wall, which you wouldn't expect to be sound proof! The weak points are doors and windows. Double glazing helps, but use a panel that has different thicknesses of glass (so they don't both resonate at the same frequency), and you may need a further secondary unit (different thickness again). Door needs to be very substantial (I built one on staggered stud design - a two man lift) and multiple seals around the edges. Alternatively have two well sealed doors with the wall thickness air gap in between.
Cavity walls are not as good as you might think, and are probably now the weak point in my set up, in particular the inner skin transmits the sound to adjacent rooms in the house. However no complaints from neighbours, so I'll leave those alone.
My solution to the ventilation problem is to take a break and leave the door open when ever it gets too stuffy.