A sound treatment assembly for use in a fluid transmission line, the sound treatment assembly comprising: an outlet member extending along a longitudinal axis from an open first end to a longitudinally opposite open second end, and an outlet bore extending from the first end to the second end, the outlet member having a first flange portion disposed at the first end, the first flange portion being adapted to be coupled to a portion of a valve body of a regulator valve, wherein the first flange portion includes a non-planar contoured mating surface adapted to mate with a corresponding contoured portion of an exterior surface of the valve body, the outlet member having a second flange portion disposed at the second end; a diffuser assembly comprising: a diffuser housing extending along a longitudinal axis from an open first end to a longitudinally opposite open second end, the diffuser housing having a bore extending from the open first end to the open second end, the bore including a first portion that extends from a first end to a second end, wherein the first portion diverges from the first end to the second end, the bore including a second portion that extends from the first end of the first portion to the first open end of the diffuser housing, the second portion of the diffuser housing has a constant cross-sectional shape; one or more diffuser elements (155, 157, 159) disposed within the bore of the diffuser housing, characterized in that the diffuser assembly further comprises: a first flange portion disposed at the first end of the diffuser housing, the first flange portion disposed adjacent to the second flange portion of the outlet member; a second flange portion disposed at the second end of the diffuser housing, the second flange portion adapted to be coupled to a portion of a downstream pipe. an outlet member extending along a longitudinal axis from an open first end to a longitudinally opposite open second end, and an outlet bore extending from the first end to the second end, the outlet member having a first flange portion disposed at the first end, the first flange portion being adapted to be coupled to a portion of a valve body of a regulator valve, wherein the first flange portion includes a non-planar contoured mating surface adapted to mate with a corresponding contoured portion of an exterior surface of the valve body, the outlet member having a second flange portion disposed at the second end; a diffuser assembly comprising: a diffuser housing extending along a longitudinal axis from an open first end to a longitudinally opposite open second end, the diffuser housing having a bore extending from the open first end to the open second end, the bore including a first portion that extends from a first end to a second end, wherein the first portion diverges from the first end to the second end, the bore including a second portion that extends from the first end of the first portion to the first open end of the diffuser housing, the second portion of the diffuser housing has a constant cross-sectional shape; one or more diffuser elements (155, 157, 159) disposed within the bore of the diffuser housing, characterized in that the diffuser assembly further comprises: a first flange portion disposed at the first end of the diffuser housing, the first flange portion disposed adjacent to the second flange portion of the outlet member; a second flange portion disposed at the second end of the diffuser housing, the second flange portion adapted to be coupled to a portion of a downstream pipe. a diffuser housing extending along a longitudinal axis from an open first end to a longitudinally opposite open second end, the diffuser housing having a bore extending from the open first end to the open second end, the bore including a first portion that extends from a first end to a second end, wherein the first portion diverges from the first end to the second end, the bore including a second portion that extends from the first end of the first portion to the first open end of the diffuser housing, the second portion of the diffuser housing has a constant cross-sectional shape; one or more diffuser elements (155, 157, 159) disposed within the bore of the diffuser housing, characterized in that the diffuser assembly further comprises: a first flange portion disposed at the first end of the diffuser housing, the first flange portion disposed adjacent to the second flange portion of the outlet member; a first flange portion disposed at the first end of the diffuser housing, the first flange portion disposed adjacent to the second flange portion of the outlet member; a second flange portion disposed at the second end of the diffuser housing, the second flange portion adapted to be coupled to a portion of a downstream pipe.