When i bought the car the only sound insulation was a single layer of badly fitting carpet and the car was very noisy – which i thought was normal as never owning an mgb before.
I put 10mm sound insulating felt under the Carpets and 20mm self adhesive foam around the sides and top of the foot wells. Over the front carpets i had bought some rubber over mats that would also add to the sound insulation.
The difference in sound levels was impressive. At low speed there is still some back axle wine as building up speed. At normal speeds the car is as quiet as any modern car but at speeds over 50 then wind noise is the most prevalent issue.
So overall i am quite pleased with the results from adding all the insulation.