Fantasonics (www.fantasonics.com) do an excellent set of CDs with scale noises. I bought Big City (1940s) and Big Port/Harbour (1940s) CDs on a two for one offer.
I bought some cheap (under a fiver) portable CD players from ebay and some cheap speakers from Maplins:
I cut them up and extracted the speakers and internal gubbins. The speakers needed enclosures for which I used paint can caps (Halfords primer caps were ideal but I didn’t have enough so I robbed all my spray cans with large enough caps). I cut a small notch for the wires and then I glued them on with bathroom silicone sealant:
The entire set up has two speakers, a volume control with on/off switch and extra earphone jack and a power converter. They are mains powered and so I need to run some extension leads around to reach the power sockets as the cables are not long enough to reach and I need to mount the CD player somewhere that the play button can be reached to turn them on.
I have Big City playing on two CD players to give it more spread along the street area. I have a random setting on the CD player which means that the tracks don’t repeat and clash. The speakers for the Big City were hidden under the elevated section. The first set point out of the back into open space so that they will reverberate around the city area. There will be a building across the front.
The next set were hidden in a more open area of the elevated section but there are also Tortoise motors to hide.
I am proposing to use buildings to hide them like the mock up below: