A sound insulation in tubular parts regardless of the section shape of the part, uses a piece of material that is suitable for expanding over the entire section of the section, which piece is mounted using a support suitable for fixing to the wall of the tube that is to be insulated. In a particular embodiment, a sound insulation assembly mounted in a part presenting a longitudinal direction and a tubular wall comprises a wafer that is thermally expandable to form a foam, said wafer having two parallel main faces and extending essentially along a main plane from a first wall portion towards the diametrically opposite wall portion, together with a wafer support comprising a clamp for retaining the wafer prior to expansion and coupled to fixing means in the form of a clip for fixing to the first wall portion of the part.