We have a beam in a wall between two rooms. I want to fill in the space with some form of insulation to contribute towards overall acoustic insulation between the the rooms.

 

What would be best for this. I have some leftover PIR that would fit but I suspect I am better using some acoustic wool batt?

Yes, mineral wool is far better than pir for this.

PUR, PIR & polystyrene have no acoustic properties. Mineral wool should help here but if this is a beam supporting a wall above and bearing on another wall below flanking transmission may be in issue.

Seal any air paths too with acoustic caulk.