IMPO This unit is not very loud compared to other brands of sound decoders. Opened it up and found two things that are an easy fix.
1 the hood and sound escapement is terrible. Sat the engine on track, while idling, lift the cover, noticed 2 or 3 times the sound volume. Cut holes on top of hood and is hid by the radiator screen, can hardly tell the modification.
2 The speaker enclosure had two holes that Athearn ran the truck wires through to the decoder. Removed the truck wires and ran them under the decoder board, glued styrene cover over flywheel. The two holes in the speaker enclosure, lets the sound waves mix from the fromt and back, this is not good, just this made it sound better.
hmm interesting modification, on my mp15ac i got a lot better sound by mounting the speaker to the body and ensuring the speaker was sealed. improved the sound ten fold and it was an easy modification. good to see your mod worked for you, well done.