Digitrax
SoundFX(TM) Soundbug(TM) Sound-Only Decoder

SFX004 Plug N' Play Sound Generator Module w/28mm Speaker
Walthers Part # 245-SFX004, p. 804 Walthers 2012 HO Scale Reference
HO scale, $49.99, currently in stock at Walthers
This product is on-sale today for $44.98

 
Digitrax Soundbug Module:
 
Plug N' Play with any 165 series decoder Have Smart Power Management - no more booster or programmer shutdowns! Can function as a standalone stationary trackside/under-layout sound decoder With fully customizable Digitrax SFX sound capability Includes 28mm 32 Ohm Speaker & 330uF Capacitor Comes with steam or EMD SD38-2 sound files; other sound files (projects) are downloadable at digitrax.com for customizing sounds to fit your favorite loco.