The Aristo SD-45 comes with a speaker which is mounted on four posts on the radiator fan assembly.  The speaker points upward, which is a plus, because the sound is directed in that direction.  The speaker is an 8 ohm one with a fairly heavy magnet on the rear which should project some bass tones really well.  However the front and rear of the speaker are open to the inside of the body shell which permits all sort of sound leakage which diminishes the volume that you would normally hear.  The ends of this speaker are entirely open as well, further degrading the quality and volume that the unit is capable of producing.  As I stated in previous sound system and speaker articles, the speaker in a model locomotive must be in some sort of enclosure so that most of the sound is directed out of the front of the speaker to the listener.
       One of the ways to fix this problem in the SD-45, is to build a set of walls out of 0.125" x 0.250" styrene to totally enclose the three radiator fans on the inside of the body shell.  Seal all the gaps in these walls with clear silicone. The speaker is then mounted on its original mounting posts.  Two covers are then made from 0.080" styrene and glued over the top of the walls on either side of the speaker.  If needed, seal the edges of these covers where they meet the speaker ends with clear silicone.  You have now essentially built  a small speaker enclosure that will direct all your sound out all three radiator fans, with the long hood now acting as an enclosure for the speaker.
       This installation should increase your speaker volume, and will also enhance the bass effect that you are looking for in your diesel prime mover.