As Pete suggested earlier, speakers are somewhat tricky. It's not only size and power handling capability, but efficiency as well. Since a majority of the small speakers we use don't have good spec's available, some of this is left to experimentation.
Note that there are two volume settings on the RS Commanders, try hitting AUX1 and then volume up and repeating that command sequence a few times and see if that doesn't bring the background sounds up.
I have an Alco PA RS Commander on the bench right now, and the prime mover sounds seem plenty loud in mine. They even louder with an 8 ohm ERR speaker than the 16 ohm that came with the kit.
