Hearing protection devices (HPDs) are designed to reduce the amplitude of sound reaching the ear, but can also distort the acoustical cues necessary for sound localization. In order to quantify the acoustic impact of an HPD on the signal reaching the tympanic membrane, the signals deep in the ear canal with and without the HPD must be compared. While some commercial microphone solutions for measuring in-ear pressures with HPDs exist, they are device-specific and/or may not provide reliable isolation of the microphone transducer from the (potentially high-intensity) ambient sound field. Here, we describe a new assembly using a small MEMS microphone attached to the end of a thin flexible circuit, which can be inserted either alongside or through virtually any HPD. Following a description of the electromechanical design of the assembly, we present measurements of insertion loss for HPDs with and without the microphone using an acoustic test fixture. These measurements demonstrate that the presence of the microphone does not significantly disrupt the sound attenuation characteristics of a range of HPDs. Preliminary measurements of insertion loss and head-related transfer functions in human subjects are also presented.