I'm not familiar with that locomotive at all. A couple months ago I installed a Loksound Nano hardwired into a couple of P2K S2 locomotives with a small sugar cube speaker just before the cab.
If you could post a couple pictures I could probably give you some ideas.
Pete.
This is a DCC with sound ready loco. Should not be too difficult to find a small decoder & speaker to fit into it. Look at the Bachmann parts break down of the loco to give you some ideas.
Thanks for all who replied. For this engine, I have decided to have someone, who has a lot more skill than I do, install a decoder for me.
According to the parts diagram, the DCC ready S4 has an opening where a round speaker goes on the sound-equipped version. You can buy the speaker from Bachmann if you want, part #10080.
https://www.bachmanntrains.com/home-usa/dwg/dwgs/HO_ALCO_S4_(DCC_READY).pdf
Keep in mind the sound-equipped ones aren't all that expensive if you shop around online - it may turn out that buying a sound one and swapping bodies could be cheaper than buying a decoder, speaker, and paying someone to install them for you.