From speaking to Gerry about his MIR 121s 
there is simply not enough room in the body 
for a speaker, mainly because of the gear towers 
and height of the motor and twin fly wheels used 
on the Athearn SW 1500 chassis that were normally 
used on the MIR 121 body's.Obviously it would great 
to use the 141 sound chip in the 121 body, and hope 
any future MM 121 loco will have room for a speaker. 
I am not sure as regards any US switchers that might 
have sound on board that might fit the MIR 121.