For ac external duct ( on the roof )- did we have to insulate the inside of duct to reduce the sound or not ?

For external ducts there is no need for sound insulation because normally we are not concerned with sound on roof moreover external ducts are heavily insulated and most of the times have Jacket as well.

It depends on sound calculation result if it is suitable for the space sound level or not (calculation will depend on the pass of the sound)

- some project you will find it indicated in the specs before and after any equipment

- one project i have 15 m length duct lining before and after any equipment, another one is 3 m.

- you can also apply sound attenuator or silencer

Based on my experience it is not healthy to use Insulation from Inside. Because there is a chance of top surface of Insulation to be damage and fiber of Fiber Glass may transfer to space with high velocity air (Usually starting duct are high velocity). If there is a problem of sound better solution is Sound Attenuator.

Depending of the Authority Having Jurisdiction Government Department as Fire Department, or the Engineering Plan of the Installation of the AC Duct, I don't think so, is not necessary at the time, but is always good to as at the appropriate jurisdiction of the country to find what is best before finishing the complete installation, as long you have an approve installation plan with the Engineering seal, your good to go.

Noise created inside the duct is negligible when compared to noise developed by other equipments installed in roof . Moreover compliance is required as per the project spec and acoustic report recommendation if any .Sound attenuator is more enough in supply and return main duct  but in exhaust duct it is not mandatory to provide internal acoustic liner.