What you are trying to do--want to do--is to run two different decoders off the same physical DCC Address. The (N-scale) motor+light decoder is in a locomotive, and a separate sound decoder (HO size or even O size would work fine) which is NOT in the locomotive, but located somewhere in, under, around your control panel (and Zephyr). You want to send the speaker output from the sound decoder (which would normally go to an onboard speaker), to a bluetooth transmitter instead...and then send that audio to wireless bluetooth speakers. Yes?
The sound decoder doesn't have to fit inside a locomotive or tender, so you don't have to have it onboard at all. It doesn't need to be small, either--a HO sound decoder would work fine.
I would hardwire the sound decoder it to the DCC track bus, connect the two speaker lines to a plug which would connect to an off the shelf bluetooth transmitter like this one:
If you scan the pics for that item, you see it can take inputs using RCA plugs or a 3.5mm audio plug, and you connect the other end of that cable to the two outputs from the sound decoder.
Your scheme makes sense in that the speaker itself is already adequately powered, so you don't have to boost a hardwired speaker yourself. True too if you connect one of your speakers by wire.
I won't start listing decoders--the list is very long--but your sound decoder only needs to control the speaker. Any motor/light signals would not be connected, as the onboard decoder already does that. Lok-sound does have some, but other brands will too.
It's not that different from the way some steam engines are set up: Motor+light decoder inside the cab or boiler (because that's where the motor is), and sound decoder in the tender, where you can output to a speaker. Both on the same address.