Surround Traxx from sound traxx - Model Railroader Magazine

Here you go: 

I hope it helps.  I saw a demo of the system at PSX 2004 (NMRA Train Show) in N-scale and I was duly impressed with the sound quality.

It seems to me that Surroundtraxx is the only viable solution for those in N and Z-scales who want sound.  Just be aware that each loco has to have a decoder capable of transponding, and that you will need a number of Digitrax transponder repeaters to make it realistic.  I don't remember if the system supports 4 or 6 speakers.

Yes, Surroundtraxx could get expensive but bear in mind that you will spend a minimum of $100 each loco in which you install sound.  That includes the decoder, the speaker, and materials (solder, plastic for enclosures, etc.)  

I've done 15 of these so far (and am hopefully finished with my fleet) and my best record is around 8-10 hours[ some take the better part of 24 hours if any frame milling is involved.  That includes the planning time to figure out how to make room for the decoder and actually wiring it in.  The biggest time consumer is figuring out where the speaker can go.

There are some installation where the decoder is a simple plug-in but there is still the issue of the speaker installation.  Some of the newer locos include space for a speaker.  That helps a lot.