While I have DCC decoders installed in nearly all my locomotives only 1 has a sound decoder. It's this Atlas SD7 which has an early MRC sound decoder. The frame was milled to be able to get the speaker in. Something I really did not like was how with any brief interruption of pickup from the track, the sound would cut out then start up again, plus the sound quality was not very good with the tiny speaker. So this loco ended up sitting on a siding with it's idling sound running.
At some point the technology of electrical capacitors developed into what are called a "super capacitor" which has a large amount of capacitance in a small size. Adding such circuits to a DCC sound decoder installation eliminates this problem of the sound cutting out but it's still a challenge in N scale to find room for decoder, a speaker, and a super capacitor circuit.
For my next attempt at DCC sound, I chose to build a sound car. This was built around an Atlas covered hopper car and used a Soundtraxx decoder and a "keep alive" super capacitor circuit from Train Control Systems. Here is a LINK to a short Youtube video demonstrating this project. And here is another LINK to this project on my DCC blog.
When I installed the grade crossing signals in Carlin, I used an 8 ohm, 3 was speaker that I found on Amazon for about $6.00 and was small enough to fit into a house that was next to the grade crossing. Here is a LINK to the post I made on that project. I really liked the sound quality from this speaker.
About the time I had bought the Soundtraxx decoder I had also purchased a Digitrax SFX0416 sound only decoder with the idea to build a sound dummy locomotive with a Kato SD40. I never got around to this project but one day was wondering if this decoder could drive one of the 3 watt speakers and how that would sound. The decoder was able to drive this speaker and sounded better than anything I've heard from a speaker in an N Scale locomotive. So it was decided that I would use this sound decoder and this speaker as a stationary setup to add sound to the Carlin yard switcher. The next post will cover the details of how I did that.