QSI used to sell an upgrade kit to add sound to the "compatible" engines. The upgrade kits are easy to install, but very difficult to find, and often expensive if you can find an NOS one. The Scaled Tin Rail used to have a few but they wanted a lot of money for them. Since the death of the previous owner I don't know what happened to those kits or if the new owners still have them for sale. I don't know if it's possible to use leftover PS-1 parts to upgrade the "compatible" system. It certainly would be practical to drop in a complete PS1 setup from an engine that's been upgraded to a command system, and used PS-1 setups can be found for 50 bucks or less, with all the electronics and the speaker. Personally I would either leave it as is with the horn and bell or upgrade to TMCC with an ERR kit, rather than bother with PS-1. Of course, you could also do PS-2 but those kits can be hard to find and they can cost more than list price when they are out of stock at MTH, which is usually.
One note on QSI reverse boards - they all are short on power handling capacity. Don't run long trains for long periods of time with ANY QSI or PS-1 reverse board. The diodes will overheat. The Scaled Tin Rail used to do an inexpensive diode upgrade that would cure the problem, but I don't know if the new owners are still doing that or not.