+1 to Mr Beasley
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
Pretty much the only "sound only" decoder on the market is the Digitrax Sound Bug. It's relatively inexpensive, at $50 MSRP and just over $40 discounted. The Sound Bug comes with a generic diesel and generic steam sound scheme, selectable by setting a CV. If you (or a friend, club or LHS) has a programmer for it (the Digitrax PR3, typically) you can download sounds from Digitrax and put that into the Sound Bug. But, you get what you pay for. You also get a speaker.
At the other end of the reasonable scale is probably the Tsunami from Soundtraxx. Tsunamis must be bought with the sound you want already installed. They are sound-and-motor decoders. And, they sound much, much better than Sound Bugs. But, you'll pay for them, and for the speaker, which is not included.
In your situation, I would buy one of each and install them. For the Tsunami, you'll be removing the original motor decoder and letting the Tsunami handle that, too. No big loss on the Bachmann. It's not a very good decoder anyway, and the motor control of the Tsunami is far superior.
Once you've got one of each, you can make the call. I'm thinking your ears are going to overrule your wallet.
It takes an iron man to play with a toy iron horse.
I have 13 DCC equipped Bachmann locos for which I want to install "sound only" decoders. Naturally, price is a major consideration. Two locos representing all the others are the GE Bethlehem #60605 and the Spectrum 8-40CW #83506. Any help with identifying compatible sound decoders would be greatly appreciated.