Plug-n-Play decoder, non-Plug-n-Play Loco?

[P5se Camelback]

Those are some scary-tall gear towers! They must come up to the bottom of the cab windows?
Not a lot of clearance over the top in the hood for a speaker there, I think. Therefore I think you're going to need to go with a deck in the cab just over the top of the gear tower, and put a speaker over/under it (depending on looks vs. clearance). What's the fuel tank look like? Any chance of making that your speaker enclosure?

Figures! It sat all apart on the "breakfast area work bench" all afternoon while I went to renew Horace Dodge's insurance (it fell due while I was in the hospital -- as did Horace's registration tag - now expired -- tomorrow's task) and came home, put it back together, moved booksheves to an area henceforth known as the Library to make room for the Weissport leg of the pending layout, did a breathing treatment, ate some supper and sat down here.

O.K. ... it sits on the end table next to me now. (BTW ... the horn no longer plays skyward!) The cab end towers are short of the cab window by about the thickness of the screw heads, the tops of which sit even with the window sills. I'll have to wait until I get the decoder (next month ... hopefully) and see if there isn't some room for a speaker to be stolen above one of the drive shafts, or maybe between the drive shaft and the floor -- I'll have to get out the dial-face calipers and take a bunch of measurements ... it'll be tight, but most of the time, ten or fifteen thousanths is all the clearance you need to squeeze something in and still have room for things to fullfill their assigned destiny ... after all, driveshafts only rotate!

Fuel tanks ... HA!... take a look! We have two good-sized battery boxes and a pair of cylindrical tanks hanging in between them. From the associated piping, they might be the fuel tanks ... whaddya say Charlie B.? Are they the fuel tanks or are they air tanks and the fuel tank is hiding somewhere else?

No, I'll just have to take my measurements now and maybe mock up some speakers based on published dims and see what happens. Fear not ... I'll keep you posted ... and I'll photograph each step along the way, because, being one of us, I know we need pictures!