i have no experience with that loco, nor with the 3s-21, but given those provisos, could you try this:

1) save the 3s-21 for another project and purchase a 3S.
2) remove the printed circuit board and all the white connectors, identifying all the wires (track current, motor leads, lights, etc)
3) hard wire the 3S to all the wires that were previously connected to the now-removed circuit board
4) use the smallest speakers you can find (i've used iphone 4 speakers which are pretty thin)

if none of this is applicable to your model than feel free to ignore the advice!

good luck

Ken

ps--if you are still short on space in wedging in a 3S, you can remove the plastic housing (voiding the warranty) and wrap the module with Kapton tape which will provide electrical insulation.

Bummer, I was so hoping this was going to work... 

Carl

Yeah - sorry about that guys!
I had typed up a response (saying I didn't have a C:\ProgramData option) and posted it, to only then see that I did indeed have that C:\ProgramData option - DUH! 
It was late, I deleted that post (almost instantly) since it was false. Didn't think it would help the historical record to have incorrect information posted. I'm amazed that Tim saw it before I deleted it!
Being as late as it was, I told myself I'd deal with it tomorrow (today).
So, suffice it to say, I did find that "RscLoaded" file just as Tim suggested, deleted it, and HC Simulator now appears to start up and run fine. I have not fully put it through its paces yet, but, I'm optimistic! If anything changes, I'll report back.
Paul

Yeah, the compressor seems to be from a GE.  I believe I told Tim this in the past but nothing came from it.  I would prefer to have the compressor from the 567 file placed on the GEVO and 7FDL-16.  It's worth bringing it back up to Tim again.

I'll need to give the 645-12 another listen.  I like the 567 ULT even though it is definitely stack heavy. 

Compressor, yes. Thanks, I have yet to do any work with sound files.