I'm not familiar with these and noted no help via examples at the TCS install pages which are often good for real examples. I do see that there are diagrams for BLI NW2 and SW1500, which show provision for a speaker. Don't know if this is your version and for sure whether a DCC model w/o sound has the speaker room available for sure. But I'd bet if willing to remove the board and hard wire, you could get a LokSound Select (or Micro) as well as a round speaker as shown (or sugar cubes).
http://www.broadway-limited.com/bluelineexplodedviewdiagrams.aspx
A call to BLI or Tony's Trains could give you specifics on what will work easily on your particular model, unless someone here in the forum has the speciics you need.
Paul
Modeling HO with a transition era UP bent
Has anyone tried installing a sound decoder in this small switcher, this loco comes with a factory NCE ver 3.6 decoder,and runs well.but i would like to replace it with a sound decoder,i have not disasembled the loco as of now,but i would like to get some info from the communty befor i take on the challenge.