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A material that absorbs sound is called a sound-absorbing material.
Literal
Sound [object-を] absorb material [genitive-の] thing [object-を] sound-absorbing-material [quotative-と] call.
A technical definition. Structured as a standard ~のことを~という pattern for introducing a technical term ('X is called Y'). 吸音材 ('sound-absorbing material') is a construction/engineering compound: 吸 (absorb) + 音 (sound) + 材 (material). The relative clause 音を吸収する ('that absorbs sound') modifies 素材 ('raw material, substance') — note that 素材 is more general than just 'material' in a craft or design sense. The parallel 吸音材 and 吸収する素材 are essentially the same thing, with the compound as the technical term for the descriptive phrase.