The inside of the model is accessed by removing the coupler boxes and four screws under the chassis (one at each corner). There is a lighting wire connecting the body to chassis, so take appropriate care when removing the body.
The decoder is inserted as shown below, noting the 21 pin socket on the decoder faces DOWN, such that the decoder is mounted UPSIDE DOWN. This is not normal practice. The front face of the speaker faces up, as shown below. Seal any air gaps between speaker and enclosure to deliver best sound. MicroScale KristalKlear is a suitable product.
Custom X class speaker from DCCSound specifically designed for the X class as shown below. The assembly simply replaces the conventional 16x35 speaker and Auscision enclosure and delivers much richer, louder sound.