With respect to the voltage management issue in going from command track voltage 18V AC down to 5V DC; and you need several Watts of power to drive an audio system.
The AC-DC module that Matt shows above would be nice...but can you fit it?! Note if you plan to use this on a DCS system you need to add the 22uH choke before the bridge rectifier. And if you have some chokes from a rolling stock LED project, you may need to re-visit the power capability since the current going thru the inductor will be much higher in this application.
Another option which unfortunately gets complicated real quickly is to use a "mini" DC-DC converter module. These are apparently popular for r/c airplanes. Search eBay for "mini DC buck converter." For example here's a side-by-side of a typical LM2596 buck converter module vs. one of these mini modules.
Several tedious details. Obviously this is DC-DC so you need to add your own bridge-rectifier. The mini module does not have a large input capacitor so you need to add that too. And of course you need the 22uH choke for DCS operation.
Another problem is the allowed input voltage to the mini module. Some say 24V DC maximum. So if coming from full command voltage through a bridge rectifier, the DC voltage can/will exceed 24V DC. So some voltage dropping is needed - not rocket science (a few diodes for example) but another tedious detail to manage. Note that most/all LM2596 DC modules handle 30V DC or more on the input.
I'm simply suggesting the idea of breaking down the problem to deal with space/fit issues. Here's a recycled photo from another blog where I put the bridge, choke, and capacitor in a compact assembly...followed by a DC-DC module. The separate elements gives some flexibility perhaps fitting different components where there's a nook or cranny.
Here's the entire blog post which has additional pictures
http://www.jcstudiosinc.com/Co...-Using-12V-LED-Strip
