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.
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
How do sound only decoders work?
Can they be used in conjunction with an already installed, non-sound decoder?