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I would suggest finding a locomotive that's DCC and sound equipped since it's your first. Fewer headaches. Then as you get more experienced you can go the pick and choose route...
For a Kato N SCALE SD70Ace Locomotive I would suggest an MRC sound decoder for ease of installation mainly due to the speaker being attached to the Decoder board..
Unfortunately there does not seem to be an MRC sound decoder for a Kato SD70Ace, only for a SD70 MAC and AC4400, which is a part No 1960 decoder..
If your layout is not that big (i.e. not filling an entire room) then have you considered under layout sound?
I have two Kato SD40-2 locomotives fitted with MRC sound decoders, and the sound from them is well, umm, rather tinny, and after a while I find it somewhat annoying, and I often turn the sound off.. With such a small speaker you don't get the full rumble sound of a locomotive, just a high pitched whine...
SO, what I plan to do is install "under the layout sound" on my 3 metre x 1 metre layout using a MRC HO scale sound only decoders and some PC speakers.. To make it work, I will either just give the sound decoder the same address (road number) as the locomotive that I plan to use OR give the sound decoder its own unique address, and then do a basic or advance consist the sound decoder with the locomotive I want to have sound..
My layout is 11 by 6, L shape. You can see it on another thread I started a few weeks ago. If you are referring to something like I seen on soundtraxx that is a lot of money. I would need to see and hear something like that firsthand before jumping into something like that bu tthanks for sharing the thought.