I don't know if they could be mounted stationary,

    I think any decoder could be stationary if you wanted, it just needs to be connected to the rails.  And of course it needs a speaker which takes up room in any car but if a stationary mounting the speaker could be really big.  Think I should try that, but doing a quick look at their site I'm not sure how the sounds are activated.  If they were simple F key stationary would work, if the decoder needs to be consisted then that may be a problem.

-- 
Jon Miller
For me time stopped in 1941
Digitrax--Chief/Zephyr systems, JMRI User
NMRA Life member #2623
Member SFRH&MS