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.