How do sound only decoders work?
Can they be used in conjunction with an already installed, non-sound decoder?
Yes or you could do one of 2 other things.
Replace the non sound decoder with a sound one or install a sound decoder with motor controller but not connect the motor.
I would definitely remove the standard decoder and replace it with a QSI or Tsunami. If you install a separate decoder the sounds may not be properly synchronized. Also things like ditch lights that work with horn may not work correctly.
Springfield PA
I have a few sound-only decoders. They are all made by Digitrax. I have one in a Bachmann trolley that came with a motor decoder, and another in a dummy F7A unit that has no motor. They are OK. Not great, but OK.
The SoundBug, the original Digitrax sound-only decoder, is fine for what it does. The SFX0416, on the other hand, has very limited volume and can barely be heard, even over other non-sound locomotives which might be running.
I have another dummy F7 that I may put a SoundBug in, but my current choice for sound upgrades is the Tsunami. I think the overall sound quality is so much better, and by installing a single sound-and-motor decoder, I avoid programming and synchronization problems, and I simplify the wiring inside the locomotive, too. I've put in one diesel and one steam Tsunami, and I very happy with both of them
It takes an iron man to play with a toy iron horse.