Blocking up a double door sized doorway

You've already answered your own question, timber, fibreglass and p-board...just make sure the timber + two thicknesses of p-board are the same as the existing wall, screw the pb on for rigidity rather than nails, cavity batts are probably easier for working with than loft insulation and more dense too.

HTH

Yes but you can minimise this by using scrim at the edges

As others have said, inserting some studding and covering it with plasterboard is fairly easy.

Another alternative is simply to throw up some blockwork, and then get a plasterer to plaster it. That will probably make a better job of sound-proofing, and you're less likely to get cracks in the plaster where the new bit joins to the existing brickwork.

If you - or a subsequent ower - want to restore the doorway, demolishing the blockwork is not really any more difficult than demolishing a stud partition.

I assume that there's a lintel over the doorway - so you won't have to worry about your solution needing to support whatever is above it?