Insulate void between 2 flats - best type of insulation to eliminate sound

hello - i want to insulate the void between 2 flats. There is about 12inches between groundfloor plastboard ceiling and upper floor floorboards. The main objective is to eliminate sound rather than heat/energy.

There are so many types of insulation, which is best for this job? Also considering that there are pipes and wires in this void, is there a suitable insulation that comes in rolls or will it have to be boards cut up?

Should I leave a gap above and below the insulation?

Any advice would be welcome. it's one of those jobs that you have to get right first time.

Thanks