A case for a portable electronic device includes an enclosed, elongated space extending within the case from an opening in an interior surface of the case. The opening corresponds to the location of a microphone in the device. The enclosed space is tapered to an increasingly smaller cross-section as it extends away from the opening. A first edge of the enclosed space aligns along most of its length with an exterior surface of the case. A screen covers a second opening in the exterior surface of the case along the first edge of the elongated space and provides an acoustic resistance between the elongated space and air outside the case through the second opening. A gasket located at the first opening couples the elongated space to the microphone of the electronic device.