I purchased a Legomanbiffo DCC sound decoder to install into the Bachmann class 205 DMU. The process of installing the sound chip was very straight forward, in fact the most challenging part of the project was separating the carriage body from the chassis without damaging any of the delicate components.
The instructions advised that two screws needed to be removed underneath the driving cab end of the chassis but my particular example of the model featured only one single screw, this still had to be removed to release the chassis from the body. Once the body was removed, an area of the passenger seating was detached (circled) in order to accommodate the sound speaker.
The original Bachmann 21 pin DCC decoder was removed and replaced by the Legomanbiffo sound decoder with the speaker installed in the seating area. I wasn’t sure if it was better to install the speaker facing up or down, my personal preference was to install the speaker facing up.