Z scale decoder with stationary sound decoders

I have been working on a Z scale coffee table layout and want to add sound. Yeah I know, not enough room in the engines. My plan is to install sound decoders in the table and program them to match my various decoder equipped locos. The layout is under glass and only 34x36 so no need for zones or surround sound. To get the best sound I would prefer a larger speaker like those in HO scale setups. However to be able to use a larger scale decoder I would most likely have to wire them in ahead of my voltage reducer. Will this work or will there be sync issues?

I am using Digitrax throttles with both Digitrax and TCS decoders. I would like to have sound one decoder for every engine Any one done this and have any tips to share?

What we have done is install the sound decoders on boxcars and passenger cars. The AZL high cube works very well, but we have sound cars installed on a MTL 50' boxcars. By the way, we don't drop our voltage at all, but make sure you have short protection.

I picked up some Digitrax Soundbug decoders and programmed them to match the lead engines I am running. This works pretty well, just some adjusting with the CV's to get better synchronization to do. One thing that bugs me is the relatively quite sound output that is drowned out by the engine/wheel noise. I changed the CV to set sound to max and am working on a better speaker enclosure, but I think what I really need is a bigger speaker possibly run through an amp.
Does anyone know of any ready made amps to convert speaker output to line in, or at least a kit to make one myself?

Looks like I may not need an amp. I built a proper speaker enclosure and the sound is audible even under the glass. I am going to add 3 more Sound bugs for now. 1 GP30, 1 GP38, 1 SD40-2, and 1 SD70. I am still running with the stock SD38 loaded and it sounds pretty good. What I really need is an Alco and a C30-7 to get some variety for my sound. When I convert my F59 and Mikado to DCC they will each get one. 6 Sound decoders is still cheaper than a Surround Traxx.