Hi,
I've been following this thread for some time thoroughly enjoying the developments and the skill of your work. I come here for inspiration for my own modelling efforts.
On the topic of sound I have a relatively small depot layout and a number of sound locos but it's true that you only ever operate one or two at a time and often the remainder sit silent. The loco sounds certainly do bring a whole new dimension though and I have found it expensively addictive. As my layout is early 70s Western Region themed I have a number of locos that don't even function having purchased cheap Hornby Hymeks, and Mainline Warships for example, at swap meets, eBay and the like, removed their dodgy motors and modelled them in scrap-line condition - sadly they are just sitting there rusting away slowly.
Now the last thing you need is being side tracked from your efforts but another thing I have done was as a result of something I read in one of the modelling magazines a couple of years back. This was to create sound for inside my main depot building. Using a cheap MP3 player and a small speaker also found on eBay I created a sound file of depot noises that emanate out of the building where I hid the speaker. There are a number of sounds that can be obtained and downloaded from the internet. The main source I used was Audacity at www.audacityteam.org. I created my own sound file with air compressors, shouts, clanging of tools, hammering, ringing phone, air tools, drills and the like all occurring at random intervals. It worked so well that I repeated the exercise for the small street scene running along the back of my layout with the occasional parping horn, motorbike, car, bus noise in dispersed by bird noises.
So don't let me distract you from your current endeavours (cause we all love seeing your update pictures) but maybe something for when you've run out of things to do!!!!!! I was really pleased with my results and it adds yet a further dimension - I certainly considered it worth it when I'd finished.
Just an idea.