If it's a DCC sound decoder and you want it
to be stationary, simply connect the input to
your DCC bus, give the decoder a 'spare'
address, and you can control it with your
reguar DCC controller.
I like Lemonhawks suggestion, if you have a
specific loco you want to be 'sound' simply add
the stationary decoder to a consist with that
loco. I would think it would offer the same
sound that it would if on board 'that' loco.
My only question, would the stationary decoder
need a 'load' in place of a motor?
I guess you could connect an old used motor
to it and just let it whir under the table.