Don't want to give too much away at the moment, but this is something I am working on for my new layout - saw a great example of just that at last year's Newcastle show and it worked well.
Dubbieside had sound added on the main line around the back and general shunting sounds going on in the docks below. The back one worked from my phone and it was attached around the back to a pair of speakers hidden behind the back scene. Complete trains were recorded running past with the correct locos and formation passing by on the upper main line. I liked to operate it when on duty behind the scenes. It was better than simply locos with chips as you had the complete sound of the train running by. My own favourite was of an A4 and train which passed steadily by, but after disappearing from the scene could be heard sounding its chime whistle in the distance.
Don't know if you have seen another friend Mike Walshaw's depiction of a military camp railway, but it is excellent and sound is distributed around the layout depending on where the locos are working. Mike had a specific system built for his layout. It also has deep sounds and sounds wonderful. In America most of the lads I know have had sound chips for years fitted to their locos. The last time I was across they were discussing new chips that had come on to the market there on to which you could add the sounds yourself - cost at the time $25! Why are we paying so much for chips in this country?
Could I suggest that you consider joining MERG as loads of sound things are available via them - you do have to do your own assembly - but I will be posting on some of the kits in a couple of months when I get round to trialling them for my own layout. There are several ways of doing what you want by the way - but if you wait as I say we will maybe go into all that in hopefully Nov/Dec if all goes well with my own layout building.
Don't want to distract from Colin's thread as I am enjoying it and am pointing others to it who are going through the same stage at the moment in our Starters Group. I am off now to cut some wood.
in electronics, but most of the modules are fairly straightforward to construct and use. They are also fairly cheap so a few mistakes will not blow the bank - they do take time to put together, but not so bad if doing batches (a bit like wagons), but assembly can have its own fascination and you will probably have MERG members nearby and gain a few new friends into the bargain.
) I used the white metal castings (MSE) which were available - they were only going to be seen at a distance. Since it was 1883 I assumed that the Stevens rods were round tube - they produced tubing for gas originally and what I used was fishing line glued in with superglue. This may seem a strange choice, but it has worked and stayed in place despite rough treatment from time to time. 
However I am still impressed when anyone manages to get such like things working. Howard's mechanism is an interesting possibility. I have no end of tales that can be told about the various mechanisms on Burntisland or getting my ash plant and tippler to operate on Dubbieside - maybe something to discuss on another thread sometime. Good luck. Back to the carnage of F1!
