If you get a really small speaker as designed for smaller scale locos, there is room to hide one of up to about 20-22mm diameter inside the little coal bunker on the left side of the cab. This comes off with one tiny screw under the footplate, and is about the only useable enclosed space on the loco. The other place to put the speaker is to fix it under the cab roof, facing downward, with a simple "enclosure" made from blu-tack - I've done this with a Stainz, no reason it shouldn't work for the Feldbahn too.
Actually, if you use an M decoder in the front compartment for the driving unit, and run some extra power wires back to the cab, there is JUST enough space to squeeze a Massoth S sound unit into the aforementioned coal bunker, if you snap the unwanted end tabs off the S unit's PCB. Then put the speaker in the roof as above.
Jon.